DTMF transmission in softphone mode

DTMF transmission between ProCall Enterprise and PBX

DTMF stands for Dual Tone Multifrequency, also known as MFV for multi-frequency procedure.

estos ProCall Enterprise offers the option of transmitting DTMF (or DTMF) characters.

In the article Answering incoming calls – the communication window the way to show the DTMF keypad is shown via the item Select DTMF tones in the conversation window.

Most telephone systems connected to estos ProCall Enterprise support DTMF signaling. Please refer to the list of supported telephony functions published for the respective PBX.

Whether DTMF character transmission is actually supported in the context of a SIP softphone call is signaled to each other by ProCall Enterprise and the PBX on the basis of the SDP information exchanged with each other. In estos ProCall Enterprise, there is therefore no configuration option for switching DTMF support off or on.

The DTMF codec information (m-line) exchanged during the call setup must agree on the codec telephone-event/8000 to have the DTMF functionality available during the call:

Example of a session description for an INVITE request 

Here is an example of a session description for an INVITE request from the telephone system to the estos UC media server:

v=0
o=OsBiz 1 93456798 IN IP4 10.20.20.100
s=on transit
c=IN IP4 10.20.20.100
t=0 0
m=audio 30270 RTP/AVP 8 0 97 18 4 104 13 105 101
a=rtpmap:8 PCMA/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:97 iLBC/8000
a=rtpmap:18 G729/8000
a=rtpmap:4 G723/8000
a=rtpmap:104 G726-32/8000
a=rtpmap:13 CN/8000
a=rtpmap:105 telephone-event/16000
a=rtpmap:101 telephone-event/8000

In this call, the PBX offers the DTMF codecs telephone-event/16000 and telephone-event/8000 to the estos UC media server.

On Connect, the estos UC media server of the PBX responds with the codec telephone-event/8000 that it supports and has selected:

v=0
o=- 1687527440784 1687527440784 IN IP4 10.20.30.200
s=UC Media Server
c=IN IP4 10.20.30.200
t=0 0
m=audio 62974 RTP/AVP 8 101
a=sendrecv
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000

If no DTMF codecs are offered by the PBX for the SIP call via m-line, a successful negotiation of the DTMF payload type is not possible and the DTMF functionality is not available in estos ProCall Enterprise for this call. Here is an example of exchanged session descriptions for a call where DTMF is not available:

estos UC Media Server gibt beim Aufbau des SIP Dialogs den DTMF Codec telephone-event/8000 (Payload Type 97) vor:

v=0
o=- 1687502937558 1687502937558 IN IP4 192.168.52.97
s=UC Media Server
c=IN IP4 192.168.52.97
t=0 0
m=audio 35794 RTP/AVP 96 9 8 0 97
a=rtpmap:96 opus/48000/2
a=rtpmap:9 G722/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:97 telephone-event/8000


Die Telefonanlage antwortet im Session Progress / 183 und im OK / 200 ohne Angabe eines DTMF Codec, sondern nur mit den Sprachcodecs G.711 aLaw (Payload Type 8) und G.711 µLaw (Payload Type 0):
v=0
o=OsBiz 1 311880504 IN IP4 192.168.52.60
s=SIP Call
c=IN IP4 192.168.52.60
t=0 0
m=audio 30256 RTP/AVP 8 0
a=rtpmap:8 PCMA/8000
a=rtpmap:0 PCMU/8000