Just a shot in the dark...make sure the codec being used on the line for those calls is compatible with the codecs on the gateway or SIP trunk, whichever you are using for outbound calls. Also make sure that if you are using G.729 you are not running out of licensed resources.
That's what first comes to mind. I saw RFC3261 documentation regarding SIP "warning code" 304 being a media type mismatch. You should check the SIPEngine trace log as well for that code.