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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.

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  • \$\begingroup\$ I think we still use pulse dialing, but I'm not really sure. \$\endgroup\$
    – Vladimir Cravero
    Commented Aug 18, 2015 at 12:04
  • \$\begingroup\$ @VladimirCravero Pulse dialing is only used for land-line connections, it was needed for old electro-mechanical phone exchanges. GSM is a cellular network which only transfers bits (data) over the network. So te detect what someone is dialing you would have to decode and extract these bits. I am unsure if this is even possible as the data is also encrypted. \$\endgroup\$
    – Bimpelrekkie
    Commented Aug 18, 2015 at 12:13
  • \$\begingroup\$ @VladimirCravero Wow, a pulse dialling cellphone network would be some sort of crazy steampunk fantasy. \$\endgroup\$
    – Nick Johnson
    Commented Aug 18, 2015 at 14:48
  • \$\begingroup\$ Well I thought so because I sometimes hear some clicking noise that reminds me of an old pulse dialing phone. \$\endgroup\$
    – Vladimir Cravero
    Commented Aug 18, 2015 at 19:20

1 Answer 1

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The dual-tone multi-frequency (DTMF) system of transmitting data via. standardized audio tones is baked into the GSM standard and works similar to that of a land-line. When a call is in progress, pressing a number button generates a combination of audio frequencies based on which button is pressed (seen here.) If you have the ability to receive and interpret raw voice data over the GSM band, then you just need to listen for these standard tones and decode them into button presses. A quick search turns up example code libraries for doing just that, in addition to pre-built modules if you wanted something a little simpler.

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  • \$\begingroup\$ Thank you! DTMF was the keyword I needed. Very interesting! \$\endgroup\$
    – user147133
    Commented Aug 18, 2015 at 13:06

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