Many mobile phone and “voice modem” formats are 2-, 3-, 4-, 5-, 6-bit mono PCM at various odd sample rates … the formats are binary so there’s often no way to know what the encoding is by looking at a file, unless someone has already given it a specific extension as a clue (e.g. .G721, .G726, .GSM etc.). WaveLab won’t import these, and your audio interface almost certainly won’t be able to play them without conversion, so I use Awave Studio for such jobs.