@Edward Velo:
Of course it would be possible to do that. But remember conventional telephones with their low-quality microphones and speakers would not be able to reproduce such high frequencies.
Apart from that, it had to be a codec with very low complexity (to avoid long delays). MP3, AAC or something like that were not applicable. So they just took the "normal" PCM Codec (which does in fact not perform any psycho-acoustical data reduction, it just uses a non-linear digitization). The data rate is, compared to more complex codecs like AAC, Speex or something like that, rather high (in relation to the sampling frequency)..
Audio bandwith of 16 kHz would require a sampling frequency of 32 kHz, which would cause unneccesarily high traffic using a normal PCM codec.
Best regards,
kzuse