asterisk users - Aug 2003 - GSM file format

Hi folks,

We're getting some IVR/busy messages recorded for Asterisk which I
understand have to be in GSM format but we've been given a list of options
which don't mean a lot:

RAW GSM 6.10 audio stream
RAW 'byte aligned' GSM 6.10audio stream
US ROBOTICS VOICE MODEM W.O. HEADER
US ROBOTICS VOICE MODEM W. HEADER

I've looked in the archive but can't see anything.

Can anyone help?

Thanks,
Simon