Generally speaking in order to modify the outgoing SDP content for SIP messages the application must be build and running in 3PCC mode. See chapter 5 here:
www.dialogic.com/.../globalcall_for_ip_hmp_v12.pdf
Are you running in 3pcc mode. If not, and its 1pcc then you will not be able to have access to making such changes as its controlled by the SIP stack only.
Jeff
