How to differentiate if a call is a mobile or an IP call?

Question

I'm working on a telephony app and I need to check if a call is mobile or IP. It doesn't matter if this check is done on incoming calls or on active calls. I'm testing the IP calls with WhatsApp and Skype.

I saw another answer saying to use this method:

public boolean isVoip(Context context){
    AudioManager manager = (AudioManager)context.getSystemService(Context.AUDIO_SERVICE);
    return manager.getMode() == AudioManager.MODE_IN_COMMUNICATION;
}

However, manager.getMode() == AudioManager.MODE_IN_CALL both for mobile calls and IP calls.

I've also looked into the TelephonyManager documentation and found the getPhoneType() method.

mTelephonyManager = (TelephonyManager) mContext.getSystemService(Context.TELEPHONY_SERVICE);

But mTelephonyManager.getPhoneType() returns TelephonyManager.PHONE_TYPE_GSM instead of TelephonyManager.PHONE_TYPE_SIP, even when making / receiving IP calls, so I've figured getPhoneType() returns the phone type for the current device.