2. Echo Cancellation
Try to disable or enable the Echo Cancellation on the port settings. Specically for the FXS/GSM/FXO ports.
And see if the issue persists.
3. FXO Mode
If you don't know which one to choose. You can try FCC, or TBR21 (For European countries) https://en.wikipedia.org/wiki/TBR21
4. GSM Module Detection Method
5. Feature Code Impact
6. SIP DTMF Mode
Try to check the DTMF mode for the call by following this article: Understand the DTMF in SIP Call
Then configure the proper the DTMF mode for SIP extension or SIP trunk according to your practical case.
7. SIP DTMF event compatibility
8. Mobile Audio Coding