I find that I frequently dont receive incoming calls - I am in testing phase because I need to ensure that the setup works.
I use VOX FTTH. I have installed Zoiper on an iPhone 6 running iOS 12,x. I can make outgoing calls no problem. I intermittently receive incoming calls. When I phone with my native Freshphone number (0872226227)or my Geo ported freshphone number 021 851 3216, either there is just no response from the network and the call terminates, or the call just rings on the calling side, but not on the receiving side.
When Zoiper gets closed by your iPhone it will Deregister. You can purchase Zoiper Pro which has Push Notifications to keep the app alive when moving to the background on your iPhone.