convert file for asterisk

example

sox foo.wav -r 8000 -c 1 foo.gsm resample -ql
sox foo.wav -r 8000 foo.gsm resample -ql
sox foo.wav -r 8000 -c1 foo.gsm resample -ql
sox foo.wav foo.gsm

Note: To convert wav file to gsm format. Easiest is convert wave to PCM at 8000 Hz, 16 bit, mono then use sox command as sox foo.wav foo.gsm

Links
Asterisk sound files
Convert WAV audio files for use in Asterisk