DTMF

DTMF (Dual-Tone Multi-Frequency) is the signaling used by telephone keypads. Each press transmits two simultaneous tones, one from a row frequency and one from a column frequency. Decoding pairs the two numbers back into the original keypad character.

Only the 16 keypad characters are valid plaintext: digits 0-9, letters A-D, *, and #. Anything else (letters E-Z, spaces, punctuation) is silently dropped during encryption since no DTMF tone pair exists for it.

             1209 Hz   1336 Hz   1477 Hz   1633 Hz
   697 Hz       1         2         3         A
   770 Hz       4         5         6         B
   852 Hz       7         8         9         C
   941 Hz       *         0         #         D