When you want to convert typical MP3/WAV files for your PBX like Asterisk, you can use sox and lame to do it easily.


lame --decode somemp3file.mp3 output.wav

First, convert the MP3 to WAV file.

Then,
For u-law:

sox -V output.wav -r 8000 -c 1 -t ul result.ulaw


For GSM:

sox output.wav -r 8000 -c 1 result.gsm resample -ql


Pre-converting your input sound files usually give much better results than using internal modules for transcoding.