I have been using different types of sip account to receive calls. I have tried

vbuzzer.com
localphone.com
gizmo/sipphone
sipgate

50% of incoming calls to these accounts on my N900 doesnt ring the phone. Most of the time they go to voicemail or something like that. Sometimes, they ring but no voice.

Tried with different Wi-Fis and Tmobile 3G. Still the same.

Reflashed the device also, no change

Any idea?