DTMF stands for Dual Tone – Multi Frequency and it is the basis for your telephone system. DTMF is actually the generic term for Touch-Tone (touch-tone is a registered trademark of ATT). Your touch-tone® phone is technically a DTMF generator that produces DTMF tones as you press the buttons.
Many public safety pagers and control systems use audio tones to activate. The most common type are two tones that are sent in sequence. Hence it’s names, Two-Tone Paging or Two-Tone Sequential.