calls are being immediately answered and then dropped on first ring

Question

I have tested my setup with Linphone & VoipFone clients, both of which work fine.

However when using Zoiper Pro any incoming call are now being immediately answered (I don't have auto answer on) and then dropped so I'm unable to answer any incoming calls.

Any idea how I resolve this given that two other apps work fine in the same location, on the same devices and the same network configuration ??