I know it's a few months ago since this post, however I fixed mine pretty easy. I have a Verizon phone on a T-Mobile network. I set up the VPN how it should be etc etc.
Outgoing calls: Seldomly worked but 90% of the time it would dial out then a few seconds later hang up without ringing.
Incoming Calls: perfectly fine!
Fix: swipe down from the very top of your screen. Click on settings icon > advanced calling > advanced calling turn on/off HD voice and video call.> Turn off this setting. And Voila! Back in business.