Is a the most popular media container format on phones without the support of 3G, which is currently somewhat outdated. Its main advantage is a compactness. Video data here gets stored in h.263 or MPEG-4 formats. AAAC-LC or AMR-NB are available for audio.
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.