Stopping & starting music on incoming calls

There are a few things you can do:

First of all, you can listen for changes in the call state using a PhoneStateListener.
You can register the listener in the TelephonyManager:

PhoneStateListener phoneStateListener = new PhoneStateListener() {
    @Override
    public void onCallStateChanged(int state, String incomingNumber) {
        if (state == TelephonyManager.CALL_STATE_RINGING) {
            //Incoming call: Pause music
        } else if(state == TelephonyManager.CALL_STATE_IDLE) {
            //Not in call: Play music
        } else if(state == TelephonyManager.CALL_STATE_OFFHOOK) {
            //A call is dialing, active or on hold
        }
        super.onCallStateChanged(state, incomingNumber);
    }
};
TelephonyManager mgr = (TelephonyManager) getSystemService(TELEPHONY_SERVICE);
if(mgr != null) {
    mgr.listen(phoneStateListener, PhoneStateListener.LISTEN_CALL_STATE);
}

Remember to unregister the listener when it’s no longer needed using the PhoneStateListener.LISTEN_NONE:

TelephonyManager mgr = (TelephonyManager) getSystemService(TELEPHONY_SERVICE);
if(mgr != null) {
    mgr.listen(phoneStateListener, PhoneStateListener.LISTEN_NONE);
}

For more information read the documentation.

Another thing you can do is listening for the broadcast android.intent.action.PHONE_STATE. It will contain the extra TelephonyManager.EXTRA_STATE which will give you information about the call. Take a look at the documentation here.

Please note that you’ll need the android.permission.READ_PHONE_STATE-permission in both cases.