Thank you for your attention Ivan. Yes, it happens when it is in background, that's the problem.
All the problem stops when I turn off "force background'.
As you know how IOS works, without this option Zoiper is useful just to make calls. I need a softphone just to receive them. Without that option the only way (I know) to make it works is to let the Iphone unlocked with the app open. Is there another way?
My Voip service provider support said I have to use UDP and advised me to change the Audio setting from software to audio. In my tests the problem keeps happening.
Today I could realise a problem with Duolingo too. Besides the fact that Zoiper makes Duolingo stops every sentence in the middle, it also decreased the audio quality of that app.
I'd like to take the opportunity to let you know another issue. As I am testing simultaneously the "call receiving process" I can tell that even when the "force background" is turned on, sometime it doesn't work. At least, not in the correct moment. I'll try to explain better: I tried to call my Voip from a landline while I was playing duolingo. Zoiper didn't ring at that time, but just when I closed Duolingo. The same problem could be observed with RSSRadio, for example.