How to hang up outgoing call in Android?

Capturing the outgoing call in a BroadcastReceiver has been mentioned and is definitely the best way to do it if you want to end the call before dialing.

Once dialing or in-call, however, that technique no longer works. The only way to hang up that I've encountered so far, is to do so through Java Reflection. As it is not part of the public API, you should be careful to use it, and not rely upon it. Any change to the internal composition of Android will effectively break your application.

Prasanta Paul's blog demonstrates how it can be accomplished, which I have summarized below.

Obtaining the ITelephony object:

TelephonyManager tm = (TelephonyManager) context
        .getSystemService(Context.TELEPHONY_SERVICE);
try {
    // Java reflection to gain access to TelephonyManager's
    // ITelephony getter
    Log.v(TAG, "Get getTeleService...");
    Class c = Class.forName(tm.getClass().getName());
    Method m = c.getDeclaredMethod("getITelephony");
    m.setAccessible(true);
    com.android.internal.telephony.ITelephony telephonyService =
            (ITelephony) m.invoke(tm);
} catch (Exception e) {
    e.printStackTrace();
    Log.e(TAG,
            "FATAL ERROR: could not connect to telephony subsystem");
    Log.e(TAG, "Exception object: " + e);
}

Ending the call:

telephonyService.endCall();

EDIT: To Android P or newer, please see: https://stackoverflow.com/a/51121175/450148

Try this:

(I used Reflection to access advanced telephony features and modify somethings)

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

try {
    //String serviceManagerName = "android.os.IServiceManager";
    String serviceManagerName = "android.os.ServiceManager";
    String serviceManagerNativeName = "android.os.ServiceManagerNative";
    String telephonyName = "com.android.internal.telephony.ITelephony";

    Class telephonyClass;
    Class telephonyStubClass;
    Class serviceManagerClass;
    Class serviceManagerStubClass;
    Class serviceManagerNativeClass;
    Class serviceManagerNativeStubClass;

    Method telephonyCall;
    Method telephonyEndCall;
    Method telephonyAnswerCall;
    Method getDefault;

    Method[] temps;
    Constructor[] serviceManagerConstructor;

    // Method getService;
    Object telephonyObject;
    Object serviceManagerObject;

    telephonyClass = Class.forName(telephonyName);
    telephonyStubClass = telephonyClass.getClasses()[0];
    serviceManagerClass = Class.forName(serviceManagerName);
    serviceManagerNativeClass = Class.forName(serviceManagerNativeName);

    Method getService = // getDefaults[29];
    serviceManagerClass.getMethod("getService", String.class);

    Method tempInterfaceMethod = serviceManagerNativeClass.getMethod(
                "asInterface", IBinder.class);

    Binder tmpBinder = new Binder();
    tmpBinder.attachInterface(null, "fake");

    serviceManagerObject = tempInterfaceMethod.invoke(null, tmpBinder);
    IBinder retbinder = (IBinder) getService.invoke(serviceManagerObject, "phone");
    Method serviceMethod = telephonyStubClass.getMethod("asInterface", IBinder.class);

    telephonyObject = serviceMethod.invoke(null, retbinder);
    //telephonyCall = telephonyClass.getMethod("call", String.class);
    telephonyEndCall = telephonyClass.getMethod("endCall");
    //telephonyAnswerCall = telephonyClass.getMethod("answerRingingCall");

    telephonyEndCall.invoke(telephonyObject);

} catch (Exception e) {
    e.printStackTrace();
    Log.error(DialerActivity.this,
                "FATAL ERROR: could not connect to telephony subsystem");
    Log.error(DialerActivity.this, "Exception object: " + e);
}

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)