A dual-tone multi-frequency (DTMF) decoder may decode DTMF symbols received in an audio input and transfer the symbols to a buffer. A volume accumulator may determine whether the audio input includes audio with volume above a predetermined threshold. The buffer may receive instructions from the timer or volume accumulator to transfer stored DTMF symbols to a filter, which may identify a characteristic of information conveyed by the DTMF symbols and send the identified characteristic information to an output device. The timer may determine a time associated with when each DTMF symbol was received and transfer each associated time to the buffer, which may transfer each associated time to a group boundary detector that may determine a predetermined number of longest time durations in between the DTMF symbols. The presence of a predetermined number of longest time durations in between the DTMF symbols may indicate a telephone number format.