How to hang up outgoing call in Android?

1. Create a BroadcastReceiver with a priority of 0.
2. In the BC intercept the ACTION_NEW_OUTGOING_CALL intent in its onReceive method
3. call setResultData(null) in the same method

This will prevent the call from initiating (as long as your receiver is the last to process the intent I think)

Here's the most updated code, which will work for Android P too, because it has an official API for it (here) :

in manifest, add this:

<uses-permission android:name="android.permission.ANSWER_PHONE_CALLS"/>

In code, use this:

Java:

@SuppressLint("PrivateApi")
public static boolean endCall(Context context) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
        final TelecomManager telecomManager = (TelecomManager) context.getSystemService(Context.TELECOM_SERVICE);
        if (telecomManager != null && ContextCompat.checkSelfPermission(context, Manifest.permission.ANSWER_PHONE_CALLS) == PackageManager.PERMISSION_GRANTED) {
            telecomManager.endCall();
            return true;
        }
        return false;
    }
    //use unofficial API for older Android versions, as written here: https://stackoverflow.com/a/8380418/878126
    try {
        final Class<?> telephonyClass = Class.forName("com.android.internal.telephony.ITelephony");
        final Class<?> telephonyStubClass = telephonyClass.getClasses()[0];
        final Class<?> serviceManagerClass = Class.forName("android.os.ServiceManager");
        final Class<?> serviceManagerNativeClass = Class.forName("android.os.ServiceManagerNative");
        final Method getService = serviceManagerClass.getMethod("getService", String.class);
        final Method tempInterfaceMethod = serviceManagerNativeClass.getMethod("asInterface", IBinder.class);
        final Binder tmpBinder = new Binder();
        tmpBinder.attachInterface(null, "fake");
        final Object serviceManagerObject = tempInterfaceMethod.invoke(null, tmpBinder);
        final IBinder retbinder = (IBinder) getService.invoke(serviceManagerObject, "phone");
        final Method serviceMethod = telephonyStubClass.getMethod("asInterface", IBinder.class);
        final Object telephonyObject = serviceMethod.invoke(null, retbinder);
        final Method telephonyEndCall = telephonyClass.getMethod("endCall");
        telephonyEndCall.invoke(telephonyObject);
        return true;
    } catch (Exception e) {
        e.printStackTrace();
        LogManager.e(e);
    }
    return false;
}

or in Kotlin:

@SuppressLint("PrivateApi")
fun endCall(context: Context): Boolean {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
        val telecomManager = context.getSystemService(Context.TELECOM_SERVICE) as TelecomManager
        if (ContextCompat.checkSelfPermission(context, Manifest.permission.ANSWER_PHONE_CALLS) == PackageManager.PERMISSION_GRANTED) {
            telecomManager.endCall()
            return true
        }
        return false
    }
    //use unofficial API for older Android versions, as written here: https://stackoverflow.com/a/8380418/878126
    try {
        val telephonyClass = Class.forName("com.android.internal.telephony.ITelephony")
        val telephonyStubClass = telephonyClass.classes[0]
        val serviceManagerClass = Class.forName("android.os.ServiceManager")
        val serviceManagerNativeClass = Class.forName("android.os.ServiceManagerNative")
        val getService = serviceManagerClass.getMethod("getService", String::class.java)
        val tempInterfaceMethod = serviceManagerNativeClass.getMethod("asInterface", IBinder::class.java)
        val tmpBinder = Binder()
        tmpBinder.attachInterface(null, "fake")
        val serviceManagerObject = tempInterfaceMethod.invoke(null, tmpBinder)
        val retbinder = getService.invoke(serviceManagerObject, "phone") as IBinder
        val serviceMethod = telephonyStubClass.getMethod("asInterface", IBinder::class.java)
        val telephonyObject = serviceMethod.invoke(null, retbinder)
        val telephonyEndCall = telephonyClass.getMethod("endCall")
        telephonyEndCall.invoke(telephonyObject)
        return true
    } catch (e: Exception) {
        e.printStackTrace()
        return false
    }
}

Considering the potential for wonderful mischief I would be surprised if this is allowed.

This thread says flatly that the API cannot end a call. Others have tried.