It's not your VOIP. It's Zoiper and I suspect you're complaining about the Android version. Mine stopped as well. Meanwhile, my desktop Zoiper 5 continued to register.
I tried a new install on my son's phone. Nothing worked. I created new sub accounts at my voip.ms and still no luck.
So I tried Bria. And I tried GS. Both worked fine using the account Zoiper would not connect. Voip.ms was aware of the situation and they thought it had to do with the push proxy but I wasn't using that on my son's phone.
Zoiper pushed a crap update on us and knocked us offline. And they are very quiet about it.
Shame on you, Zoiper. I'm a paid user, not a freebie user.