Hello, I have faced with really strange situation.
I had installed a Zoiper on a PC with Windows 10 and for some reason it has stopped to receive inbound calls recently. No matter what I do - I have allowed ports 5060 in Firewall, disabled it completely, managed advanced settings (don't use stun, rport, set custom registration expiry, etc.). I can do outbound calls, but I cannot receive inbound. I have another PC and I have tested Zoiper there just to make sure it is not related to provider. So it works there really fine with same provider using the same account without any extensive configuration.
So it looks like there is problem with Windows, Firewall, NAT, PC, I don't know. It looks like magic. I have even tried to install different software, like Jitsi with same result. The main wonderful thing is that it used to work.
So maybe here is anybody who can help me understand what the heck is going on.