Hi everyone,
I wish if this posted in the right forum. I'm going to buy DTMF decoder module to use It 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.
So I'm worrying if this DTMF decoder will not works with a GSM phone or will not work at all (even with sim800L) if this signal maybe filtered or removed by mobile network between end users (if it applicable i don't know) ??? I also want to know if DTMF should be captured in outbound calls as same as incoming calls?
I read many from google and couldn't understand well about in-band signals and DTMF improvements history.
I will take a SWAG. This use to be easy to do and because it was there were a lot of different "boxes" that could dial long distance for free (yep we use to have to pay). The phone company put all the signaling on a different channel as did the cell phone systems. I am not sure if this is your problem but you might want to check into it.
You can certainly send DTMF tones through the cell phone voice channel using a speaker, but the audio quality is so bad that detection might be difficult.
gilshultz:
I will take a SWAG. This use to be easy to do and because it was there were a lot of different "boxes" that could dial long distance for free (yep we use to have to pay). The phone company put all the signaling on a different channel as did the cell phone systems. I am not sure if this is your problem but you might want to check into it.
Yes I read about some abusive case witch cause this way you took about. Can DTMF modules decode incomming 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.
jremington:
You can certainly send DTMF tones through the cell phone voice channel using a speaker, but the audio quality is so bad that detection might be difficult.
What do you really want to do?
Yes I tried this and It's not ideal enough and It's not working well as you said. I just want to make simple home automation controlled by a simple phone call from whitelisted numbers with password and without extra apps installed.
I also done this below with a lot of confusions:
I tried to call from mobile to a landline and DTMF worked well in landline but also not with all mobiles.
Tried many troubleshooting like in/decreasing volume and check after reading this article also I checked this and this.
Tried to disable dial pad tones in android and calling any IVR service and It still work with no Issues.
Tried to call cross net or on net mobile numbers and there is no new.
Tried to enable DTMF detection on GSM phone using [AT+VTD?](https://www.multitech.com/support/resolutions/What is the command used to enable / disable DTMF tone detection_o50031) but it returns ERROR as same as SIM900's AT command AT+DDET.
The problem that GSM modules like it currently is out of stock in local stores and ordering it globally will take long time and customs fees is very high and I'm worry after buying it won't work.
It is possible with some phones (Android for sure) to send keypad DTMF tones through the voice channel, but you need to find a reliable solution to receive them. It may not be cheap.