How to diagnose 400 Bad Request

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Sometimes a provider will reject outbound calls and in a packet capture it will show a 400 Bad Request message:

SS of 400 Bad request

You can see what is causing the Bad Request error by highlighting the line and expanding the Session Initiaition Protocol and Message Header fields

SS of 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:

SS for line 014

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.