A headerless file of Dialogic/OKI ADPCM audio data commonly comes with the extension .vox. This ADPCM data has 12-bit precision packed into only 4-bits.
this format was originally intended for voice compression and is able to change the bit rates of encoded audio data. AMR is actively used for encoding and decoding voice while its being transmitted over network channels. The format allows one to consider network capacity and adapt it to channel loads. Due to this, the voice quality is practically undiminished.