I'd like to add some details for the same problem. No audio ring.
I normally forward the calls to my mobile number to a VoIP number to manage some situations.
Android dialer ring normally for 20 seconds with the audio and afterwards the call comes correctly to Zoiper, and it ring as well.
After this call, if a new call comes directly to the VoIP number, and therefore to Zoiper, there is no audio for the call. I see the call but without audio.
If I want to have again the audio I have to cancel the APP. From this point everything works well again.
I'm using Zoiper Beta 2.2.23 Library revision v2.8.66-mod. Android 6.0.1 (VFD 600).