Cisco CUCM and VCS Integration: H.323 Voice Gateways

William Bell
Vice President, Solutions and Products


Recently, I have been able to work on Cisco’s IP Video Communications (IPVC) solution portfolio based on the Tandberg acquisition. Cisco is now referring to this collectively as Cisco’s TelePresence solution.

I have a few blogs in the works on this. Today I wanted to take a little time to discuss a recent issue a customer of ours encountered.

The Stage

For sake of discussion, consider the following playing field:

Our customer was running H.323 on the voice gateway and they used Cisco’s deployment guide for integrating the CUCM and VCS platforms. They provisioned a SIP trunk between the VCS and CUCM in accordance with Cisco’s Neighbor Zone approach.

The Problem

In this configuration, the customer was able to dial the auto attendant on their MCU and access meetings using DTMF from Cisco IP phones. However, the customer was not able to pass DTMF for PSTN callers.

The root cause came down to how DTMF relay was configured on the VoIP dial-peers. The dial-peers were configured to use “dtmf-relay h245-alphanumeric”.  Since the SIP trunk to the VCS was configured for RFC 2833 support there was a conflict in DTMF relay methodology. In this specific use case, the CUCM tried to engage a media termination point (MTP) resource unsuccessfully and the DTMF failed to be relayed.

The Solution

VoIP dial-peers on H.323 gateways have several DTMF options. At a high-level:

  • Use Cisco-proprietary RTP. DTMF tones sent “in-band” with the RTP channel as voice data.
  • Using H.245 signaling or H.245 alphanumeric method. These methods separate DTMF digits from the voice stream and send them through the H.245 signaling channel instead of the media channel. All H.323 v2 compliant systems support “h245-alphanumeric”. The “h245-signal” method is optional.
  • Use Named Telephone Events (NTEs). Using NTE provides a standard way to transport DTMF tones in RTP packets according to section 3 of RFC 2833.

So, using NTE made the most sense since it allows the H.323 gateway and the VCS SIP trunk to negotiate the DTMF relay method. A sample dial-peer:

dial-peer voice 100 voip
 description cucm01
 destination-pattern 4445551212
 voice-class codec 10
 session target ipv4:
 dtmf-relay rtp-nte

After making this change, DTMF worked as desired.

2 responses to “Cisco CUCM and VCS Integration: H.323 Voice Gateways

  1. thanks for this tutorial .i would like to know how to integrate codian GW with CUCM :so in this cas we can make videoconferencing sessions via ip phone with external participants.

  2. Hello William,

    Actually I am having a similiar problem in a different setup. I have registered a Codian MCU directly to CUCM. Also I have Cisco Unified Contact Center application to provide call center feature. I have defined Contact Center IVR number as an endpoint on the MCU. When I call to the conference on the MCU, the MCU dials the IVR number. But I can not send DTMF through the MCU to the IVR. What do you think is the problem.

    I will appreciate it if you advice me how to solve the issue.

    Best regards,

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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

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Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


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He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.