Basic Call Player

BCP is a tech demo application that plays an audio file to the other party when a phone call is active.

Usage

1. Download the latest version from the releases page. To verify the digital signature, see the verifying digital signatures section.

2. Install BCP as a system app.

   For devices rooted with Magisk, simply flash the zip as a Magisk module from within the Magisk app.

   For unrooted custom firmware, the files from the system/ folder in the zip will need to be baked into the system image (or otherwise made available on the actual /system volume).

3. Reboot and open BCP.

4. Enable call playback. When enabling for the first time, BCP will ask for notification permissions on Android 13+. This is required for the background service to bring itself to the foreground when a phone call is active.

5. Select an audio file to play during phone calls.

6. To install future updates, there are a couple methods:

   • If installed via Magisk, the module can be updated right from Magisk Manager's modules tab. Flashing the new version in Magisk manually also works just as well.
   • The .apk can also be extracted from the zip and be directly installed. With this method, the old version exists as a system app and the new version exists as a user-installed update to the system app. This method is more convenient if BCP is baked into the Android firmware image.

It doesn't work!

It only works on devices that implement the telephony audio output device. Newer Google Pixel devices do, for example, because it's needed for Google Dialer's call screening feature.

How it works

The phone call detection mechanism is the same as in BCR.

The telephony output works by simply using the MODIFY_PHONE_STATE privileged system permission and creating an AudioTrack instance for the telephony output device.

Verifying digital signatures

Both the zip file and the APK contained within are digitally signed.

To verify the signature of the zip file, first retrieve the public key: 2233C479609BDCEC43BE9232F6A3B19090EFF32C. This is the same key used to sign the git tags in this repository.

gpg --recv-key 2233C479609BDCEC43BE9232F6A3B19090EFF32C

Then, verify the signature of the zip file.

gpg --verify BCP-<version>-release.zip.asc BCP-<version>-release.zip

The command output should include both Good signature and the GPG fingerprint listed above.

To verify the signature of the APK, extract it from the zip and then run:

apksigner verify --print-certs system/priv-app/com.chiller3.bcp/app-release.apk

The SHA-256 digest of the APK signing certificate is:

d16f9b375df668c58ef4bb855eae959713d6d02e45f7f2c05ce2c27ae944f4f9

Building from source

BCP can be built like most other Android apps using Android Studio or the gradle command line.

To build the APK:

./gradlew assembleDebug

To build the Magisk module zip (which automatically runs the assembleDebug task if needed):

./gradlew zipDebug

The output file is written to app/build/distributions/debug/. The APK will be signed with the default autogenerated debug key.

To create a release build with a specific signing key, set up the following environment variables:

export RELEASE_KEYSTORE=/path/to/keystore.jks
export RELEASE_KEY_ALIAS=alias_name

read -r -s RELEASE_KEYSTORE_PASSPHRASE
read -r -s RELEASE_KEY_PASSPHRASE
export RELEASE_KEYSTORE_PASSPHRASE
export RELEASE_KEY_PASSPHRASE

and then build the release zip:

./gradlew zipRelease

Contributing

Unlike BCR, this project is meant as just a fun little project to demonstrate how to play audio to the telephony output device. I do not plan on doing any updates and am unlikely to accept contributions beyond small things, like translations. Please consider coming up with other interesting use cases and adding a similar feature to your own apps instead!

License

BCP is licensed under GPLv3. Please see LICENSE for the full license text.