Register string

these basic settings should work on most SIP providers in the trunk:
in Outoing:

host=<Sip provider>
username=<Your Sip provider user>
secret=<Your sip provider password>
type=friend
nat=yes << change that to no if the server is directly connected to the net.
insecure=invite
qualify=yes

leave incoming blank

register string:

<sip user>:<sip password>@<sip server>/<sip user> or <sip user>:<sip password>@<sip server>