DTMF detection issues for VOIP calls (e.g. using key presses to acknowledge alarms over voice calls)

DTMF detection issues for VOIP calls (e.g. using key presses to acknowledge alarms over voice calls)

DTMF  (Dual-tone multi-frequency signaling - Wikipedia) is responsible for communication of key presses over the phone call.
TopView can use these key presses to allow calls to proceed, or to acknowledge alarms via voice notifications.

Here are some common issues and solutions if you suspect TopView is not able to detect DTMF during a voice call.

Verify that TopView is not seeing DTMF/key presses

TopView will log all received DTMF/key presses during a phone call.
The easiest method to test DTMF/key press detection is to make a test call from the TopView Configurator. During the test call a log is displayed and updated with information during the call. Any key press events are logged to the screen during the test call.

Common issues

  1. The firewall is blocking audio communication
  2. Poor audio quality prevents DTMF detection
  3. Set DTMF detection in VOIP system to RFC2833

DTMF tones are sent to TopView from the SIP/VOIP Server as part of the VOIP call. This means that the VOIP server/system is responsible for recognizing the DTMF tone from the recipient's phone and sending that back to TopView.

DTMF issues are usually due to one of the following:
  1. Firewalling/traffic blocking issues (this is the usual one)
    1. Check your firewalls and network to ensure ports aren't blocked. SIP transport typically happens on port 5060 and RTP (phone call media) happens on another. DTMF is usually based on call audio so the RTP ports must be open. Test to make sure audio from both sides of the call can be heard. If possible, you can use Wireshark to monitor the call, or The 3CX Firewall Checker Client Application on the TopView computer to try to expose issues.
  2. Phone/VOIP server settings and issues (e.g. the DMTF settings for the phone communicate key presses in a different way that the server expects or understands)
    1. We recommend setting DTMF detection, if possible, to RFC2833.
  3. Poor audio quality that affects the VOIP server's DTMF detection
    1. You may be able to adjust the audio codecs TopView uses to improve the audio quality.
    2. The receiving mobile device may be able to switch to, or away from, VoLTE ("Voice over LTE") to see if audio quality improves.


If you are using the recommended Grandstream UCM62xx/63xx series device, there are some other options available to you:

PBX Settings > SIP Settings > ToS tab:
Entering the key presses slowing may help. Some phones have an option to play a longer tone, this would be in the phone settings.

Cisco system (e.g. CUCM)

We recommend trying to set up for detection using RFC2833 because we've seen other VOIP systems leverage that and work, and it delivers the DTMF key presses through SIP which TopView is set to monitor.

Here are some other resources that may be helpful:

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