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New Member

RFC-2833 support in Unity?

We just finished our upgrade to CCM 5.0, and were looking forward to leveraging the improved SIP features, but may have run into an issue.

We currently use H.323 IOS gateways, which I would like to migrate to SIP, that are configured to use h245-alphanumeric dtmf. Incoming calls that must use G729 to a 3rd party SIP service cannot use h245-alpha.

I tried changing the gateway to use rtp-nte, which works if the call flow looks like this:

H323 gateway -> CCM -> 3rd party sip

but fails in this case:

H323 gateway -> CCM -> Unity 4.0(5)-> CCM -> 3rd party sip

Since Unity cannot read rtp-nte encoded DTMF. I also tried setting the H323 gateway to use both rtp-nte and h245-alpha with rtp-nte having a higher priority.

In that case Unity does read the DTMF, but monitoring the 3rd party service shows that no rtp encoded DTMF is received after a Unity initiated transfer.

Cisco Employee

Re: RFC-2833 support in Unity?

What TSP version are you running on Unity? Starting with 8.1(1) the TSP will pick up RFC2833 digits. Check out the Release Notes...

New Member

Re: RFC-2833 support in Unity?

I thought I had seen that. We are running 8.1(2) as

called for by the CCM compatibility matrix.

I guess we'll need to do some more testing with our


New Member

Re: RFC-2833 support in Unity?

I have confirmed that the Unity server in question is running the 8.1(2) TSP.

If the H.323 dial-peer is set to use rtp-nte for DTMF, Unity does not get the keypad events.

So, is there a magic incantation to actually enable the TSP to use RFC 2833, or is this a quirk with H.323 on 12.3(11)T9?

Cisco Employee

Re: RFC-2833 support in Unity?

No magic needed. In fact, some would say the TSP is a bit too aggressive in grabbing those digits, see CSCsd92207. In the 8.1(3) TSP, we'll only look for RFC2833 if the CCM specifies a non-zero RFC2833 payload type during the Skinny call setup. This was also added in an 8.1(2) ES. But if your running the released 8.1(2) then the TSP should be grabbing everything.

I'd suggest looking at either the Unity Skinny diags or a network capture of the RTP stream to verify that the gateway is indeed sending the RFC2833 DTMF. I'm doubly skeptical that it is because of your comment that the 3rd party sip device wasn't seeing them either (after Unity released the transfer and was out of the call flow).

New Member

Re: RFC-2833 support in Unity?

Well, with anything new there is a bit of digging to see what is going on.

I have one router setup with SIP trunking to CCM, using 2833, and Unity sees the digits from it. All other routers are using H323, and I moved one to using 2833, and Unity does not see digits from it.

The post transfer issue appears to be IOS MTP related. A call through the router, using SIP or H323 that terminates directly to the 3rd party service works (no MTP needed or used). If the call is routed to Unity and transfered, it is assigned a MTP and I suspect that the IOS MTP does not handle the DTMF translation to 2833. We are running IOS 12.3(11)T9

New Member

Re: RFC-2833 support in Unity?

OK, so traces show no keypad activity when using rtp-nte on a H323 dial-peer.

I setup a SIP end point to dial through the 3rd party gateway, using 2833 for DTMF, and Unity sees the keypad events.

So it looks like a H323 issue on the router.

New Member

Re: RFC-2833 support in Unity?

Is there a way to for Unity 4.0(5) to advertise RFC2833 during a transfer?

I've figured out that if I dial into our gateway, connect to CCM over a SIP trunk RFC2833 works.

If the call is directed to Unity, RFC2833 still works, as Unity sees the digits, but if I elect to transfer to another number through Unity, CCM assigns the call a MTP.

At this point no DTMF makes it to the new endpoint. I suspect the reason an MTP is assigned is due to a DTMF capability mismatch during Unity transfer.