I can't find any sources on how numbers are transferred during a phone call. I'm not talking about when dialing, but after the call is established. (like when you call an automatic bot and is given questions which you answers by typing numbers)
How are these numbers transferred? Are they transferred by some standard audio beep frequencies, or is it more complex than that?
I don't know if this is GSM specific at all, but I have an Arduino GSM shield, and I wonder if it is possible to capture these numbers in any way.