Here is a generic SIP trunk for use with VoIP providers requiring registration.
Please note you should never put anything in the Incoming Settings section.
Outgoing Settings: PEER Details: context=from-pstn host=PROVIDER.HOSTNAME qualify=yes nat=no type=peer insecure=invite disallow=all allow=ulaw username=USERNAME secret=SECRET Register String: USERNAME:SECRET@PROVIDER.HOSTNAME/DID
Having /DID at the register string will allow you to use Inbound routing based on the DID.
The choice of codec ulaw should cover most US based providers, if you are outside of US you should probably choose alaw instead of ulaw.