This technology is called DTMF.
Dual-tone multi-frequency signal (DTMF),
User signaling between a telephone and a switch in a telephone system is usually used to send the called number.
Before using DTMF signal, a series of intermittent pulses were used in the telephone system to transmit the called number, which is called pulse dialing.
The telephone sends two frequency signals to the switch at the same time, and sends key information.
An audio signal with eight frequencies is divided into two groups, four in each group. Pairs of * * * can represent 16 keys, respectively representing 0-9, #, * and other keys.