I am using VBUZZER as SIP service provider. Everything setup fine. I am able to make SIP phone calls, but not receiving SIP incoming calls. All incoming calls will go to the voicemail.
The only time to receive incoming calls is the first 1 or 2 minutes that I connected to Internet. After that, it will go to the voicemail. My phone is connected to internet all time, but Vbuzzer doesn't see my N900 registers on their server.