I assume there is no forwarding from some service provider other than
VoIP.ms that could be causing the problem, but if there is, you should investigate that.
The next thing to do is route your DID to the
VoIP.ms echo test. If the problem still occurs, you should report it to
VoIP.ms. If the problem does not occur when the DID is routed to the echo test, set your routing back to normal, then try the following.
1) Disable any SIP ALG or SIP Helper, if possible, in your router's configuration.
2) If 1) is not possible, try to set X_UserAgentPort to a random number between 20000 and 65535. Also set ProxyServerPort and RegistrarServerPort to 42872. This sometimes bypasses SIP ALGs. You may need to do this from OBi Expert Configuration, if you use OBiTALK to configure your device.
3) Connect your OBi202 directly to your modem, bypassing any router. You may need to reboot your modem for this to work. Depending on your ISP, this may or may not work.
Let us know if any of those solve the problem, and if the symptoms change. If this doesn't solve the problem, let us know what router you have.