You can verify the status of the Registration out to your SIP Trunk provider. From the home screen navigate to Status:
From here simply click on SIP Registration, under Live Information and you'll be presented with the SIP Registration state for the Provider SIP Trunk.
The following screen shots show the Registration States:
SIP Trunk NOT Registered
If the SIP Trunk is not registered and it should be, then try the troubleshooting tips here.
SIP Trunk Registered