[SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?

Andrea Puddu androjoker at hotmail.com
Tue Dec 11 15:28:21 UTC 2007

Hello guys,

I'm facing an issue when a mobile (common mobile 3G phone) tries to call an internal SIP phone through its geographical number.
The issue is that when the SBC sends the INVITE to the IP phone,  in the SDP  enclosure it  suggests these codecs:

m=audio 19570 RTP/AVP 18 8 0
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:8 PCMA/8000
a=rtpmap:0 PCMU/8000

The 200 OK of the phone (SDP part only) is:

m=audio 57956 RTP/AVP 18 8 0
a=rtpmap:18 g729/8000
a=rtpmap:8 pcma/8000
a=rtpmap:0 pcmu/8000

So the IP phone start to talk with G729 and SBC replies with G711A!!! So I can't hear the voice coming from mobile phone!!!!

I have two questions:

- Can the difference between the offer (G729) and the answer (g729) be significant? I mean the difference because the "g" is not capital in the answer
- Can the lack of the "fmtp" line in the answer cause troubles?

Thanks 1000,


Organizza le tue feste, cenoni e ritrovi del Natale con Windows Live Eventi!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sipforum.org/pipermail/discussion/attachments/20071211/4d8a44aa/attachment-0002.html>

More information about the discussion mailing list