From the CLI, I'm doing:

originate sofia/internal/1000@192.168.1.1 &playback(/home/user/william.gsm)


... just as a quick test. I've been using a combination of softphones 
and a Polycom 501 as the destination. The .gsm file plays fine in an 
application such as quicktime.  .wav, .al, and .ul seem to work fine 
with the playback command.