Try disabling SIP ALG, if the router has this option.
You may also wish to change the following on your OBi, which may assist you in working around an uncooperative SIP ALG:
Service Providers >> ITSP Profile x >> SIP
X_ProxyServerRedundancy: (Checked)
Voice Services >> SPx Service
X_UserAgentPort: 20456. (This is a random number between 20000 and 65535.)
If that doesn't work, you could also experiment with the following:
Service Providers >> ITSP Profile x >> SIP
X_DiscoverPublicAddress: (Unchecked)
Let us know if any of this helps, or changes the symptoms. If it doesn't help, let us know what router you are using.