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

Andrea Puddu androjoker at hotmail.com
Wed Dec 12 08:34:56 UTC 2007

I agree with your analysis. I've noticed that for example Cisco and Linksys phone sends only one codec in the 200 OK SDP envelope.

But... Can the sending of multiple codec in the 200 OK response be an issue?
I cannot complain with phone provider about this behaviour. 

Anyway why does the phone talk with G729 and AS talk with G711?  I'm wondering if the AS performs a check on  fmtp field in the 200 OK response .................



Date: Wed, 12 Dec 2007 16:24:05 +0800
From: baolovebao at gmail.com
To: androjoker at hotmail.com
Subject: Re: [SIPForum-discussion] Is "fmtp" line needed in 200 OK answer?
CC: discussion at sipforum.org

# the "g" and "G" have no difference. and the offer indicate that it's not G729B codec.
# I don't think the lack of fmtp in answer is the reason for issue. Maybe the ip phone should reply only one codec in its SDP. Maybe the multi codec in answer make the offer confused.

On 12/11/07, Andrea Puddu <androjoker at hotmail.com> wrote:

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,



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