Asterisk sip carrier setup

We have been trying unsuccessfully to get our sip carrier working with our asterisk pbx, it works fine from a softphone so we know our carrier settings are working. The carrier is registered and connected on the asterisk pbx, incoming calls are working as they should but outgoing fail with a forbidden error its something to do with either the dialplan or configuration but we need someone who is an asterisk expert to have a look and get it working. This is a simple project for an asterisk specialist as we have already got everything else working fine, it is just the carrier we need help with