Bandwidth WebRTC Kotlin Android Dialer Sample

A native Android calling app powered by Bandwidth's WebRTC (BRTC) SDK, demonstrating outbound PSTN dialing and live call quality monitoring.

Overview

This sample application is a single Android app module that demonstrates:

• Bandwidth RTC Integration: Uses the Bandwidth BRTC SDK to establish RTC connections and manage endpoints
• Token Fetching: Retrieves a session token from a local token server to authenticate with Bandwidth
• Call Controls: Dial phone numbers, accept/decline incoming calls, mute, speaker, DTMF, and call hold
• Call History: View recent calls and redial from the recents tab

Features

Currently Supported

• ✅ Make outbound calls from the Android app to phone numbers
• ✅ Receive inbound calls routed to the app endpoint
• ✅ Real-time audio via Bandwidth RTC
• ✅ DTMF tones during a call
• ✅ Mute and speakerphone controls
• ✅ Call history / recents tab

Prerequisites

• Android Studio (Hedgehog or newer) with Kotlin support
• Android SDK (API 26+)
• A Bandwidth account with:
  • Account ID
  • API credentials able to mint BRTC tokens
  • A configured Voice Application
  • Access to the BRTC role on your credentials
• ngrok or similar tunneling service if your token server is running locally

Getting the App

Option A — Clone and build

git clone <repository-url>
cd brtc-kotlin-sample-application

Open the project in Android Studio, let Gradle sync, then run the app target on your device or emulator.

Option B — Download a release APK

1. Go to the Releases page of this repository.
2. Download the latest .apk file.
3. Install it on your device:

adb install brtc-sample-<version>.apk

Enable Install from unknown sources on the device if prompted.

Development Setup

1. Start your token server

The app expects a token server running on port 3000 that returns a JSON object with a token field at GET /token. Start your backend server and note its address.

2. Expose the server to your device

Physical device — use adb reverse so the app can reach your local server via localhost:

adb reverse tcp:3000 tcp:3000

Emulator — this step is not needed. The emulator already routes localhost to your machine.

Remote server — if your token server is not running locally, start ngrok to expose it:

ngrok http 3000

Then use the ngrok URL as the server URL in the app's connect screen.

3. Open and run in Android Studio

1. Open the project in Android Studio.
2. Connect your device or start an emulator.
3. Run the app configuration.
4. Grant microphone permission when prompted.

4. Connect

On the launch screen, verify the server URL (default: http://localhost:3000) and tap Connect. The app will fetch a token and register the endpoint with Bandwidth RTC.

How to Use

1. Connect: Enter your token server URL and tap Connect to register a Bandwidth RTC endpoint.
2. Dial: Use the keypad to enter a phone number and tap the green call button.
3. Receive a call: When someone calls your Bandwidth number, the app will ring and prompt you to accept or decline.
4. In-call controls: Use Mute, Speaker, and Keypad (DTMF) during a call. Tap End to hang up.
5. Recents: Switch to the Recents tab to view call history and tap any entry to redial.
6. Disconnect: Tap Disconnect on the connect screen to unregister the endpoint.

Code Structure

app/src/main/kotlin/com/bandwidth/brtcsample/
├── ui/
│   ├── screen/
│   │   ├── CallScreen.kt        # Dialing, ringing, and in-call UI
│   │   └── ConnectScreen.kt     # Token server URL input and connect flow
│   └── component/
│       ├── DialpadView.kt       # Dialpad grid component
│       ├── AudioWaveformView.kt # Real-time audio level visualizer
│       ├── RecentsScreen.kt     # Call history list
│       └── StatsOverlay.kt      # In-call RTC stats overlay
├── viewmodel/
│   └── CallViewModel.kt         # Call state, SDK integration, business logic
└── service/
    └── TokenService.kt          # Fetches a BRTC token from the token server

Troubleshooting

Issue: "Failed to fetch auth token"

• Solution: Verify the token server URL in the connect screen and confirm it returns { "token": "..." } at GET /token.

Issue: Calls not connecting

• Solution: Ensure adb reverse tcp:3000 tcp:3000 is running for physical devices, or that the token server URL is reachable from the device.

Issue: No audio in calls

• Solution: Grant microphone permission when prompted. If audio is still missing, verify the device is not muted and check that the speakerphone setting is as expected.

Issue: SSL / certificate errors

• Solution: If your token server uses a self-signed certificate, see app/src/main/res/xml/network_security_config.xml and AndroidManifest.xml, which include debug overrides to allow user-installed CAs.

Learn More

• Bandwidth RTC Endpoint API reference
• Bandwidth RTC Overview
• Bandwidth Programmable Voice APIs

Questions and Feedback

Reach out to support@bandwidth.com for any questions or feedback regarding this product.

Contributing

Feel free to open issues or PRs to improve the sample. Keep changes focused and include device model, Android version, and Android Studio version when reporting bugs.

License

See the repository license file for details.