Since there are many OBi users with
VoIP.ms and no problems, I would assume that either:
1. Your OBi is somehow incompatible with your modem/gateway.
2. You have set it up incorrectly (IMO unlikely, given that you can make and receive calls).
3. The device is defective.
If you can do it easily, take the device to a friend's, with a different modem and preferably a different ISP. If you still have the issue, I'd assume (3) and contact Obihai support.
Alternatively, if you want to take a port shot at the problem, try changing Voice Services->SP1 Service->X_UserAgentPort from 5060 to 5070, and Service Providers->ITSP Profile A->SIP->ProxyServerPort from 5060 to 5080. After each change, confirm that you can still register and make calls. If not, undo the change and report what error you got.