[Users] add/rewrite a tag/parameter in SDP

Hey guys, What is the best way to modify the SDP message to always include (if not replace) the a= tags in a SDP message? My problem is specifically with g729, according to the RFC, if the parameter: "a=fmtp:18 annexb=yes" is present or missing then it is assumed that g729b (g729 with VAD) and if "a=fmtp:18 annexb=no" is present then the devices will use g729a (g729 with VAD Disabled). My problem is that some cisco gateways and ATAs behave differently (of course Cisco always doing it their way), if "a=fmtp:18 annexb=no" is missing then it assumes that g729a will be used, instead of following the RFC where it should negotiate g729b. Also, asterisk doesn't support g729b so I want to avoid using g729b everywhere in my network. So what is the best way or is there a way, to either: a) insert "a=fmtp:18 annexb=no" if is not present b) replace "a=fmtp:18 annexb=yes" for "a=fmtp:18 annexb=no" if it is present Thanks in advance!! Lenir p.s. Please see the below excerpt from RFC 3555 regarding how to specify G729. RFC 3555 MIME Type Registration of RTP Payload Formats July 2003 4.1.9. Registration of MIME media type audio/G729 MIME media type name: audio MIME subtype name: G729 Required parameters: None Optional parameters: ptime, maxptime annexb: indicates that Annex B, voice activity detection, is used or preferred. Permissible values are "yes" and "no" (without the quotes); "yes" is implied if this parameter is omitted