Answer: Please see below as the answer is self-explanatory
Explanation:
Digital phones, normally uses a keyboard, with the digits 0-9, and special characters like # and *.
When the caller presses any keyboard, an electronic circuit within the phone generates two pure sinusoids, of frequencies within the vocal bandwidth (between 300 Hz and 3 khz, roughly), which identifies uniquely (as decoded by the receiving switch) which was the key pressed.
In this way, the switch reconstructs the phone number called, separating country code, area code and the local number as it receives the tones.
This system is called DTMF (dual tone multi frequency).
