Troubleshooting DTMF Interop Failures between Genesys Cloud and Legacy PBX Trunking

How-To-Articles
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Troubleshooting DTMF Interop Failures between Genesys Cloud and Legacy PBX Trunking

What This Guide Covers

This masterclass provides a deep-dive into the signaling and media-plane complexities of Dual-Tone Multi-Frequency (DTMF) negotiation. By the end of this guide, you will be able to diagnose and resolve issues where IVR menus “fail to respond” to user keypad inputs during calls bridged between Genesys Cloud and legacy PBX systems (e.g., Avaya, Cisco CUCM, or Mitel). You will learn how to identify mismatches in RFC 2833 (In-band), SIP INFO (Out-of-band), and Payload Type negotiation.

Prerequisites, Roles & Licensing

Voice engineering requires access to low-level telephony configurations and diagnostic logs.

  • Licensing: Genesys Cloud CX 1, 2, or 3.
  • Permissions:
    • Telephony > Trunk > View/Edit
    • Telephony > Edge > View
    • Analytics > Conversations > View
  • Tools: Genesys Cloud PCAP / SIP Diagnostic Logs, Wireshark (for RTP analysis), and carrier-side signaling traces.

The Implementation Deep-Dive

1. Understanding the Three DTMF Transport Methods

DTMF can be sent in three ways, and a mismatch between the Genesys Cloud Edge and the legacy PBX is the primary cause of failures.

  1. RFC 2833 / RFC 4733 (RTP Payload): The preferred method. Tones are sent as specialized RTP packets with a specific “Payload Type” (usually 101).
  2. In-band (Audio): The tones are sent as actual audio frequencies (beep-boop) within the voice stream. This is prone to compression distortion (especially with G.729) and may not be detected by digital IVRs.
  3. SIP INFO (Signaling): The tones are sent as SIP messages (INFO) outside the media stream.

The Trap:
The legacy PBX is configured for “SIP INFO” while the Genesys Cloud Trunk is set to “RFC 2833.” The PBX sends the digit, but the Genesys Edge ignores the SIP message because it’s looking for RTP packets.
The Solution: Standardize on RFC 2833 across all legs. It is the most robust and widely supported method for VoIP.

2. Resolving Payload Type Mismatches

Even if both sides agree on RFC 2833, they must agree on the Dynamic Payload Type ID.

Diagnostic Workflow:

  1. Open a PCAP of the call in Wireshark.
  2. Look at the SDP in the INVITE.
  3. Find the line a=rtpmap:101 telephone-event/8000.
  4. If the other side responds with a=rtpmap:100 telephone-event/8000, the call will establish, but DTMF will fail because one side is sending “101” and the other is expecting “100”.

The Solution: Use the Custom SIP Trunk settings in Genesys Cloud to force the Dynamic Payload Type to a specific value (usually 101) to match the legacy PBX’s hardcoded expectations.

3. Handling “Double-Digits” or “Ghost Tones”

Sometimes a single press of “1” results in “11” being received by the IVR.

Architectural Reasoning:
This typically happens when a gateway is performing DTMF Relay incorrectly. It might be passing the audio tone and regenerating the RFC 2833 packet. The IVR “hears” the beep and “sees” the packet, counting it twice.

The Solution: Disable “In-band” DTMF detection on the legacy gateway if RFC 2833 is enabled. Ensure the gateway is configured to “Suppress” the original audio tone when generating the RTP event.

4. Troubleshooting Mid-Call DTMF Shifts

A common issue occurs when DTMF works for the main IVR but fails once the call is transferred to a remote PBX.

The Trap:
The initial trunk (Carrier to Genesys) is negotiated correctly, but the second trunk (Genesys to Legacy PBX) has a different codec or DTMF setting. During the transfer (Re-INVITE), the media properties shift, and DTMF negotiation is lost.
The Solution: Enable Media Passthrough on the Genesys Cloud trunk if possible, or ensure that the Codec Priority List and DTMF Method are identical on both the Inbound and Outbound trunks.

Validation, Edge Cases & Troubleshooting

Edge Case 1: G.729 Codec Clipping

  • The failure condition: DTMF works 50% of the time.
  • The root cause: You are using the G.729 codec with “In-band” audio tones. G.729 is a compressed codec optimized for human speech; it often distorts the pure sine waves of DTMF tones, making them unrecognizable to the receiver.
  • The solution: Switch to G.711 (ulaw/alaw) for any leg requiring in-band DTMF, or (preferably) switch to RFC 2833 which bypasses the audio codec entirely.

Edge Case 2: Silence Suppression (VAD)

  • The failure condition: The first digit is always missed, but subsequent digits work.
  • The root cause: Voice Activity Detection (VAD) is enabled. The gateway “puts the line to sleep” during silence. When the user presses a key, the gateway takes a few milliseconds to “wake up,” clipping the start of the DTMF packet.
  • The solution: Disable VAD (Silence Suppression) on your SIP trunks. The bandwidth savings are negligible in modern networks compared to the reliability issues it causes.

Official References