When you have the DTMF related issue with the lines - Analog/ISDN/GSM, you need to capture the port/channel signal to check if the DTMF has been sent or received correctly.
1. Capture the port signal.
Follow the right guide as per your lines.
- Analog/GSM: How to Get the FXO/FXS/GSM Port Signal Capture
- ISDN - U-Series&Gateways: Capture the Signal of Analog/GSM/Digital Ports
- ISDN - S-Series: How to do Port Monitor for BRI&E1 Trunks in S-Series PBX
2. Analyze the DTMF wave.
1) Use the audition tool like Audition to open the port signal capture file.
- Sample Rate: 8000 Hz
- Encoding: 16-bit PCM
Check the Frequency Analysis window selected.
2) Choose any DTMF wave.
3) Check the if there exists 2 noticeable frequency peaks in the Frequency Analysis window.
If yes, it means the DTMF wave should be fine.
If it is hard to find 2 peaks, then it is highly possible to have the DTMF detection problem.
4) Check the frequency of the 2 peaks.
- Low Frequency
Move the cursor to the peak, and it shows the frequency is around 781 Hz.
- High Frequency
Move the cursor to the other peak, and it shows the frequency is around 1344 Hz.
Note that the cursor might be not 100% precise. But the deviation is safe to identify the DTMF digit.
By comparing the DTMF frequency table, we found out the DTMF we analyze is digit 5.