I am running zoiper v3.39.1 on an iPhone 12 pro max with iOS version 15.1.1
I find that I am missing messages. My voip provider is voip.ms . I send the same SMS message to my email and I receive it all the time. this looks like a zoiper issue.
I sometimes go to my SIP account list and see spinners for their status - although this is rare.
Cheers