Out-of-Band using RTP, (RFC2833). This is usually the preferred method. Remember to set the payload type if different (e.g.: 96, 101 or 110 in some cases) so payload can be 110 also.
We can see in the INVITE that a=fmtp:18 and a fmtp:110 0-16
and in the OK we can only see a=fmtp:18
So ONLY G728 was negeotiated on this call.
A. NOT CORRECT - DTMF was not negeotiated
B. G729 was negeotiated however, we are not concerned with this codec because we are looking for DTMF
C. Payload 110 Telephone event was offered in the INVITE, but was not returned in the Ok, So this was not negeotiated.
D. The correct answer, NO DTMF exists
Correct. Even though using payload 101 is more common for negotiating NTE/DTMF, it wouldn't matter if it was instead used in this case, because the SIP response doesn't support payload 101 anyway.
There is no reason you can't use DTMF with G.729 in-band NTE per RFC2833. It's only an issue when using DTMF with G.729 in-band within the same RTP payload as the voice traffic, due to compression, which is not the case here, so B is incorrect.
We can see in the INVITE that a=fmtp:18 and a fmtp:110 0-16
and in the OK we can only see a=fmtp:18
So ONLY G728 was negeotiated on this call.
A. NOT CORRECT - DTMF was not negeotiated
B. G729 was negeotiated however, we are not concerned with this codec because we are looking for DTMF
C. Payload 110 Telephone event was offered in the INVITE, but was not returned in the Ok, So this was not negeotiated.
D. The correct answer, NO DTMF exists
Correct answer is D. Look at the m=audio line in the invite, it sends 18 & 110. The same m=audio line in the 200 OK only has 18. No DTMF was successfully negotiated.
B.Dual-tone multi-frequency signaling (DTMF), fax transmissions, and high-quality audio cannot be transported reliably with this codec.
https://en.wikipedia.org/wiki/G.729
It's wrong. DTMF requires the use of the named telephony events in the RTP payload for DTMF digits, telephony tones, and telephony signals as specified in RFC 4733.
kpml or RTP-NTE is DTMF so no Event: in SIP message = kpml or RTP-NTE
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