Problem FMG with sip server

Discription of Cause codes can be refered back in SIP cause code list : http://en.wikipedia.org/wiki/List_of_SIP_response_codes

In your case 415 means "Unsupported Media Type". Here, FMG is receiving 415 message from the remote SIP server (to which FMG is connecting). It is likely that remote SIP server has not been configured to allow codecs used by FMG's SIP Interface (Audio : G711u, Video: H.264); and therefore, call establishment is being rejected by remote SIP server.

You might want to confirm the audio codec settings of remote SIP server to which FMG is trying to connect.