Sampling frequency 8 kHz/13-bit (160 samples for 20 ms frames), filtered to 200–3400 Hz.
The AMR codec uses eight source codecs with bit-rates of 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s.
Fax machines on the other hand go up to 33kbit/s.
PS: People can produce and hear frequencies well above 3400 Hz, but for phone calls people just don’t notice clipping those frequencies. If anything it helps by removing some background sounds.
The AMR codec uses eight source codecs with bit-rates of 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s.
Fax machines on the other hand go up to 33kbit/s.
PS: People can produce and hear frequencies well above 3400 Hz, but for phone calls people just don’t notice clipping those frequencies. If anything it helps by removing some background sounds.