Sometimes a provider will reject outbound calls and in a packet capture it will show a 400 Bad Request message:
You can see what is causing the Bad Request error by highlighting the line and expanding the Session Initiaition Protocol
and Message Header
In the above example it shows that there is a SIP syntax error on line 014 of the INVITE.
You can now highlight the previous INVITE message and check line 14 to see what is causing the call to fail:
In this example the 400 Bad Request is caused by an Alert-Info
header that adds a ringtone. Either the provider can be contacted to allow this header or the ring rule can be disabled to resolve.