DTMF and SIP

Hi Guys,

Just doing my study for my CCIE Voice and I came across something that always kinda caused me issues, and I thought i would share


Right now i am studying SIP, and it is talking about DTMF-Relay.

To cut a long story short: DTMF-Relay for SIP dial-peers is RTPE-NTE, RTP-NTE is in-band signalling, meaning it is part of the RTP stream, the problem is that SCCP Phones don't understand RTP-NTE! So they cannot send or receive the DTMF tones correctly.

The way to resolve this is to either:

1. Run an MTP and enable this for the trunk for CUCM (this is your only option in CUCM 4)
2. Change the dtmf-relay method on the dial-peer using

#dial-peer voice 999 voip
#dtmf-relay


Now, the thing is, when i tried thsi with my SIP provider it failed miserably, that is because the SIP provider is expecting the digits as SIP-NOTIFY, the cisco routers have a solution to this too..

for gateways running CCME with SIP trunks, you need to configure RTP-NTE. This will convert the signalling received from the SCCP phones to Named Telephony Event (Which is an RFC supported by most SIP providers.)

Here is some more info i found:

The last blog entry covered DTMF relay. This blog entry will focus on Cisco IP Phone and Cisco SIP Trunk DTMF relay support. Cisco’s first generation of IP phones do not support RFC2833 DTMF-Relay. All digits are sent out of band in either SCCP or SIP signaling packets depending on the phone load. Cisco first generation phones are referred to as Type A phones in the Cisco Unified Communications Manager 6.x SRND (Solution Reference Network Design) guide available at www.cisco.com/go/srnd. Type A phones include the following phones:

• 7902
• 7905
• 7910
• 7912
• 7940
• 7960

Cisco’s second generation phones support RFC2833 DTMF Relay when registered with Cisco Unified Call Manager versions 5.0 and later. These phones include the following:

• 7906
• 7911
• 7941
• 7942
• 7945
• 7961
• 7962
• 7965
• 7970
• 7971
• 7975

Source; http://www.networkworld.com/community/node/30207