What are DTMF tones?

Dual tone multi-frequency (DTMF) is a technique that has been used since 1963 to dial telephone numbers, and to send commands to switching systems. Prior to DTMF, telephone systems used a rotary dial, which was considerably less efficient, and as a result, soon after replacing the rotary dial, DTMF became the industry standard. Years ago, a lineman’s handset featured a rotary dial, but all modern handsets now use some configuration of the standard DTMF keypad.

Source: Blog.T3innovation.com