Dialogic ADPCM or VOX is an audio file format, optimized for storing digitized voice data at a low sampling rate.
VOX files are most commonly found in telephony applications, as well as an occasional arcade redemption game.
The original Dialogic ADPCM paper (linked to below) does not specify or mention a recording or playback frequency; it may be at the implementer discretion.
These ICs were used on popular telephony interface cards manufactured by Dialogic Corporation for use in voicemail and similar systems.
Some early BlackBerry phones that don't support MP3 format (e.g. 7100) used that codec for sound files which had ADP filetype extension.