Yeah, they’re negotiating the same codec on both sides.
The problem is the way Cisco handles the negotiation. Here’s an example of what I see in Freeswitch:
Received from CALLED :
SIP/2.0 200 OK
Via: SIP/2.0/UDP A.B.C.D;rport;branch=z9hG4bKXZZ18p8tD03cS
Date: Mon, 23 May 2011 23:50:56 GMT
Call-ID: 69258f8c-0038-122f-19ba-000c29c18d38
CSeq: 12760849 INVITE
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, PRACK, UPDATE, REFER, SUBSCRIBE, NOTIFY, INFO, REGISTER
Allow-Events: telephone-event
Supported: replaces
Supported: sdp-anat
Server: Cisco-SIPGateway/IOS-12.x
Content-Type: application/sdp
Content-Disposition: session;handling=required
Content-Length: 311
v=0
o=CiscoSystemsSIP-GW-UserAgent 4080 7999 IN IP4 E.F.G.H
s=SIP Call
c=IN IP4 E.F.G.H
t=0 0
m=audio 17122 RTP/AVP 0 19 101 100
c=IN IP4 E.F.G.H
a=rtpmap:0 PCMU/8000
a=rtpmap:19 CN/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=rtpmap:100 X-NSE/8000
a=fmtp:100 192-194
Sent to CALLING :
SIP/2.0 200 OK
Via: SIP/2.0/UDP I.J.K.L:5060;branch=z9hG4bK4C2B6C64
Call-ID: AFADEF2E-84CC11E0-B1D4F23A-27804614@ I.J.K.L
CSeq: 101 INVITE
User-Agent: GCI
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, hold, presence, dialog, line-seize, call-info, sla, include-session-description, presence.winfo, message-summary, refer
Min-SE: 1800
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 261
Remote-Party-ID: "Outbound Call" <sip:sean@ A.B.C.D >;party=calling;privacy=off;screen=no
v=0
o=FreeSWITCH 1995227603 1995227604 IN IP4 A.B.C.D
s=FreeSWITCH
c=IN IP4 A.B.C.D
t=0 0
m=audio 24636 RTP/AVP 0 19 101
c=IN IP4 A.B.C.D
a=rtpmap:0 PCMU/8000
a=rtpmap:19 CN/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
This is not the case when the call direction is reversed. Like I said, the only difference appears to be that one direction is responding with a 183 while this one is responding with a 200.