Technology – Think like Human, Act like Machine

*A small tool that lets a script place real phone calls, talk, listen, and hang up — on a real SIM, over a real cellular network.* — ## Why we built this AI voice agents are everywhere now — and they **live and die by latency and audio quality.** A second of dead air, a stiff robotic voice, or choppy, fluctuating audio is the difference between "sounds human" and "obviously a bot." Yet before every release we were measuring those things *by hand*: dial in, read a script, listen for gaps, do it again on the next build. It didn't scale, and "sounds fine to me" is not a regression test. What we actually needed was to automate a *real* phone call. Not a VoIP call. Not a simulator. An actual call on an actual carrier — the kind that rings a normal phone, goes through the normal network, and behaves exactly like a human dialing. So we could: – **Test phone systems end to end** — voice agents, IVRs, call centers, voicemail — the way a real caller experiences them. – **Run scripted conversations** — play a prompt, wait for the other side to finish talking, play the next one. – **Record both sides cleanly, on one timeline**, so we could measure **latency** ("how long after I speak does the other side respond?") — as a number, on every build. The catch: **Android won't let an app record or inject the audio of a cellular call.** That path is locked to the system. So a pure software approach is impossible. DialF's answer is simple and a little old-school: **bridge the call audio through a real USB sound card.** The phone does the dialing; a sound card plays into the phone's mic and listens on its earpiece. Your computer drives the whole thing — and we know you'll wire your own AI agents up to do the driving. — ## Why not a programmable 4G module? It's the first thing every engineer suggests, and it's a fair instinct — a cellular module takes a SIM, speaks AT commands, and dials from a script. Cheap, headless, no human in the loop. But a module isn't a phone. It carries its own compatibility quirks and behaves in ways real handsets don't — so it can quietly alter the very thing you're trying to measure. You end up testing the module's behavior, not your users' calls. That's the crux: **a voice agent's audio path *is* the product, and a module only tests a synthetic version of it.** Your agent can sound flawless through a module and still ship stutter and echo through a real earpiece — and the module never warns you, because it was never on the path your callers actually hear. DialF drives a real phone for exactly that reason. — ## What it does DialF turns a phone into something you can script: – 📞 **Make, answer, reject, and hang up calls** — on the phone's own SIM. – 💬 **Send and read SMS**, read the **call log** and **SIM list** (dual-SIM aware). – 🎛️ **Carrier controls** — toggle voicemail, run raw MMI/USSD codes. – 🗣️ **Scripted voice conversations** — play audio prompts, and *wait for the person to stop talking* using voice-activity detection before moving on. – 🎙️ **Record the call** full-duplex — your audio (`tx`), their audio (`rx`), and a **stereo mix** (left = you, right = them), all the same length and sample-aligned (great for latency analysis). You drive all of it from one command-line tool, or from a small YAML script. — ## How it works DialF has two parts that talk to each other, plus a deliberate split between **control** and **audio**: “`mermaid flowchart TB subgraph Host["Your computer"] CLI["dialf (CLI)"] D["dialfd (daemon)"] Card["USB sound card"] end subgraph Android["Android phone"] App["DialF Phone app"] end CLI –>|commands| D D <–>|"WiFi · WebSocket"| App D <–>|"audio in / out"| Card Card <–>|"headset cable"| App App –>|"dials / answers on its SIM"| Net(("Cellular network")) “` – **Control plane (over WiFi):** the `dialf` CLI sends commands to the `dialfd` daemon, which relays them to the **DialF Phone** app over a WebSocket. This is how dial / answer / SMS / hang up happen. No audio travels here. – **Audio plane (physical):** call audio flows through a **USB sound card** wired to the phone's headset jack. The card plays *into* the phone's microphone and records *from* its earpiece. The app just routes the call to the wired headset. Why the split? Because Android blocks call-audio capture in software — so audio has to be bridged physically, never over WiFi. ### A scripted call, step by step “`mermaid sequenceDiagram participant CLI as dialf CLI participant D as dialfd participant P as DialF Phone participant F as Far end CLI->>D: run call-script.yaml D->>P: dial +1… P->>F: ringing… F–>>P: answers P–>>D: call active Note over D: call.wait_answered satisfied D->>P: play prompt (out the sound card → phone mic) P->>F: far end hears the prompt F–>>P: spoken reply (phone earpiece → sound card) P–>>D: reply audio captured Note over D: VAD waits for the reply to finish D->>P: play next prompt … then hang up “` — ## How to use it ### 1. Install the CLI (macOS or Linux) “`sh npm install -g @agora-build/dialf # or: curl -fsSL https://dl.agora.build/dialf/install.sh | bash “` Then start the background daemon: “`sh dialf service install –user # runs dialfd at login “` > On a Mac or laptop, keep `–user` — it runs as you, when you log in (needed so it can reach the sound card and mic). Use plain `dialf service install` (with `sudo`) only on a headless Linux server that should start at boot. ### 2. Install the phone app Sideload the APK on the Android phone (Android 9+): – **Latest release (default):** <https://github.com/Agora-Build/DialF/releases> – or <https://dl.agora.build/dialf/dialf-phone-latest.apk> Open it, grant phone/SMS permissions, and **set it as the default dialer** (that's what lets it place and track calls). ### 3. Pair them In the app, enter the same **shared key** as your `dialfd` config and tap **Start service**. The phone finds the daemon automatically on your WiFi (mDNS). Confirm it's connected: “`sh dialf devices # your phone should appear “` ### 4. Drive it “`sh dialf call dial <phone> +15551234 # place a call dialf call hangup <phone> # hang up dialf sms send <phone> +15551234 "hi" # send a text dialf call list <phone> –human # read the call log dialf –version # CLI + daemon versions “` ### 5. Script a conversation Jobs are plain YAML — a list of steps run in order: “`yaml – type: call.dial number: "+15551234" – type: call.wait_answered # wait for a real answer, not a fixed timer timeout_ms: 30000 – type: audio.play # inject a prompt into the call file: samples/prompt-en-1.wav – type: audio.wait_for_speech # listen until the other side stops talking end_timeout_ms: 45000 silence_duration_ms: 3000 – type: sms.send to: "+15551234" body: "thanks!" – type: call.hangup “` “`sh dialf run call-script.yaml “` `audio.wait_for_speech` is the clever bit: it runs voice-activity detection on the incoming audio, so the script moves on **when the person actually finishes speaking** — not after a guess. — ## Recording and latency If you turn on recording, every call is written as **three aligned WAV files**: – `…-tx.wav` — what you sent (your prompts), mono – `…-rx.wav` — what the far end said, mono – `…-mix.wav` — **stereo**: left = tx (you), right = rx (them), so the two voices stay separated (swap with `mix_channels: rx_tx`) They're captured on a **single clock**, so they line up sample-for-sample. That makes latency measurable: cross-correlate `tx` against `rx` and the offset is your round-trip delay. “`mermaid flowchart LR TX["tx.wav · your prompt"] –> MIX["mix.wav · stereo · L=tx R=rx"] RX["rx.wav · far-end reply"] –> MIX TX -.->|"cross-correlate"| RX RX –> L["latency = the lag between them"] “` — ## Wrapping up DialF is a thin, scriptable bridge between your terminal and a real phone. The control side is clean software over WiFi; the audio side is honest about hardware — a sound card doing what software isn't allowed to. Together they let a few lines of YAML place a call, hold a conversation, and hand you a clean recording. It runs on macOS and Linux, the CLI installs from npm, and the phone app is a sideloadable APK. If you've ever wanted to put a real phone call inside a `for` loop — that's the idea. — ## License DialF is released under the **MIT License**. **Disclaimer:** This tool is strictly for engineering use only and must not be used for any illegal purposes. The user bears all legal consequences arising from its use.

A small tool that lets a script place real phone calls, talk, listen, and hang up — on a real SIM, over a real cellular network.

Why we built this

AI voice agents are everywhere now — and they live and die by latency and audio quality. A second of dead air, a stiff robotic voice, or choppy, fluctuating audio is the difference between “sounds human” and “obviously a bot.” Yet before every release we were measuring those things by hand: dial in, read a script, listen for gaps, do it again on the next build. It didn’t scale, and “sounds fine to me” is not a regression test.

What we actually needed was to automate a real phone call. Not a VoIP call. Not a simulator. An actual call on an actual carrier — the kind that rings a normal phone, goes through the normal network, and behaves exactly like a human dialing. So we could:

Test phone systems end to end — voice agents, IVRs, call centers, voicemail — the way a real caller experiences them.
Run scripted conversations — play a prompt, wait for the other side to finish talking, play the next one.
Record both sides cleanly, on one timeline, so we could measure latency (“how long after I speak does the other side respond?”) — as a number, on every build.

The catch: Android won’t let an app record or inject the audio of a cellular call. That path is locked to the system. So a pure software approach is impossible.

DialF’s answer is simple and a little old-school: bridge the call audio through a real USB sound card. The phone does the dialing; a sound card plays into the phone’s mic and listens on its earpiece. Your computer drives the whole thing — and we know you’ll wire your own AI agents up to do the driving.

Why not a programmable 4G module?

It’s the first thing every engineer suggests, and it’s a fair instinct — a cellular module takes a SIM, speaks AT commands, and dials from a script. Cheap, headless, no human in the loop.

But a module isn’t a phone. It carries its own compatibility quirks and behaves in ways real handsets don’t — so it can quietly alter the very thing you’re trying to measure. You end up testing the module’s behavior, not your users’ calls.

That’s the crux: a voice agent’s audio path is the product, and a module only tests a synthetic version of it. Your agent can sound flawless through a module and still ship stutter and echo through a real earpiece — and the module never warns you, because it was never on the path your callers actually hear. DialF drives a real phone for exactly that reason.

What it does

DialF turns a phone into something you can script:

📞 Make, answer, reject, and hang up calls — on the phone’s own SIM.
💬 Send and read SMS, read the call log and SIM list (dual-SIM aware).
🎛️ Carrier controls — toggle voicemail, run raw MMI/USSD codes.
🗣️ Scripted voice conversations — play audio prompts, and wait for the person to stop talking using voice-activity detection before moving on.
🎙️ Record the call full-duplex — your audio (tx), their audio (rx), and a stereo mix (left = you, right = them), all the same length and sample-aligned (great for latency analysis).

You drive all of it from one command-line tool, or from a small YAML script.

How it works

DialF has two parts that talk to each other, plus a deliberate split between control and audio:

flowchart TB
  subgraph Host["Your computer"]
    CLI["dialf (CLI)"]
    D["dialfd (daemon)"]
    Card["USB sound card"]
  end
  subgraph Android["Android phone"]
    App["DialF Phone app"]
  end

  CLI -->|commands| D
  D <-->|"WiFi · WebSocket"| App
  D <-->|"audio in / out"| Card
  Card <-->|"headset cable"| App
  App -->|"dials / answers on its SIM"| Net(("Cellular network"))

Control plane (over WiFi): the dialf CLI sends commands to the dialfd daemon, which relays them to the DialF Phone app over a WebSocket. This is how dial / answer / SMS / hang up happen. No audio travels here.
Audio plane (physical): call audio flows through a USB sound card wired to the phone’s headset jack. The card plays into the phone’s microphone and records from its earpiece. The app just routes the call to the wired headset.

Why the split? Because Android blocks call-audio capture in software — so audio has to be bridged physically, never over WiFi.

A scripted call, step by step

sequenceDiagram
  participant CLI as dialf CLI
  participant D as dialfd
  participant P as DialF Phone
  participant F as Far end

  CLI->>D: run call-script.yaml
  D->>P: dial +1...
  P->>F: ringing…
  F-->>P: answers
  P-->>D: call active
  Note over D: call.wait_answered satisfied
  D->>P: play prompt (out the sound card → phone mic)
  P->>F: far end hears the prompt
  F-->>P: spoken reply (phone earpiece → sound card)
  P-->>D: reply audio captured
  Note over D: VAD waits for the reply to finish
  D->>P: play next prompt … then hang up

How to use it

1. Install the CLI (macOS or Linux)

npm install -g @agora-build/dialf
# or:  curl -fsSL https://dl.agora.build/dialf/install.sh | bash

Then start the background daemon:

dialf service install --user      # runs dialfd at login

On a Mac or laptop, keep --user — it runs as you, when you log in (needed so it can reach the sound card and mic). Use plain dialf service install (with sudo) only on a headless Linux server that should start at boot.

2. Install the phone app

Sideload the APK on the Android phone (Android 9+):

Latest release (default): https://github.com/Agora-Build/DialF/releases
or https://dl.agora.build/dialf/dialf-phone-latest.apk

Open it, grant phone/SMS permissions, and set it as the default dialer (that’s what lets it place and track calls).

3. Pair them

In the app, enter the same shared key as your dialfd config and tap Start service. The phone finds the daemon automatically on your WiFi (mDNS). Confirm it’s connected:

dialf devices        # your phone should appear

4. Drive it

dialf call dial   <phone> +15551234        # place a call
dialf call hangup <phone>                  # hang up
dialf sms  send   <phone> +15551234 "hi"   # send a text
dialf call list   <phone> --human          # read the call log
dialf --version                            # CLI + daemon versions

5. Script a conversation

Jobs are plain YAML — a list of steps run in order:

- type: call.dial
  number: "+15551234"
- type: call.wait_answered      # wait for a real answer, not a fixed timer
  timeout_ms: 30000
- type: audio.play              # inject a prompt into the call
  file: samples/prompt-en-1.wav
- type: audio.wait_for_speech   # listen until the other side stops talking
  end_timeout_ms: 45000
  silence_duration_ms: 3000
- type: sms.send
  to: "+15551234"
  body: "thanks!"
- type: call.hangup

dialf run call-script.yaml

audio.wait_for_speech is the clever bit: it runs voice-activity detection on the incoming audio, so the script moves on when the person actually finishes speaking — not after a guess.

Recording and latency

If you turn on recording, every call is written as three aligned WAV files:

…-tx.wav — what you sent (your prompts), mono
…-rx.wav — what the far end said, mono
…-mix.wav — stereo: left = tx (you), right = rx (them), so the two voices stay separated (swap with mix_channels: rx_tx)

They’re captured on a single clock, so they line up sample-for-sample. That makes latency measurable: cross-correlate tx against rx and the offset is your round-trip delay.

flowchart LR
  TX["tx.wav · your prompt"] --> MIX["mix.wav · stereo · L=tx R=rx"]
  RX["rx.wav · far-end reply"] --> MIX
  TX -.->|"cross-correlate"| RX
  RX --> L["latency = the lag between them"]

Wrapping up

DialF is a thin, scriptable bridge between your terminal and a real phone. The control side is clean software over WiFi; the audio side is honest about hardware — a sound card doing what software isn’t allowed to. Together they let a few lines of YAML place a call, hold a conversation, and hand you a clean recording.

It runs on macOS and Linux, the CLI installs from npm, and the phone app is a sideloadable APK. If you’ve ever wanted to put a real phone call inside a for loop — that’s the idea.

License

DialF is released under the MIT License.

Disclaimer: This tool is strictly for engineering use only and must not be used for any illegal purposes. The user bears all legal consequences arising from its use.

Category: Technology

DialF: Drive a Real Phone From Your Terminal