Decoding DTMF if tones is not hearable

[Mamdouh-Freelancer]

Hi everyone,
It's not an issue for SIM800L itself. I'm going to buy it as soon as being sure that it will work.

I also will use DTMF decoder module with GSM phone (AT commands support), Before getting started I tested a normal call to check if DTMF tones will be received between the 2 parties but I couldn't hear any tones or keypad pressing and tested with many phones smart and old.

I read many from google and couldn't understand well about in-band signals and DTMF improvements history.

Can SIM800L/DTMF modules decode incoming signals through that different channel? Or it means any DTMF project can't be done at all even using Sim800L or any else? Is this forbidden only in my country's mobile operators so?

I was working as Call center representative and remember there were some calls I received and I asked the customer to lock his screen because I heard annoying dial pad tones and this happened too while a normal call with a friend about one year ago.

I also done this below with a lot of confusions:

1. I tried to call from mobile to a landline and DTMF worked well in landline but also not with all mobiles.
2. Tried many troubleshooting like in/decreasing volume and check after reading this article also I checked this and this.
3. Tried to disable dial pad tones in android and calling any IVR service and It still work with no Issues.
4. Tried to call cross net or on net mobile numbers and there is no new.
5. Tried to enable DTMF detection on GSM phone using AT+VTD? but it returns ERROR as same as SIM900's AT command AT+DDET.

Thanks in advance.