asterisk users - Oct 2008 - changing from default codec

hi, using sip, my default codec is set to gsm in sip.conf

I occasionally want to send out a call using ulaw while other channels
are using gsm, how can I do this using call files ?

I couldn't find any codec parameter in the call file definition.

tia.

hello,

 I am using sip, my default codec is set to gsm in sip.conf

 Using call files, is there a way to send out a call using
 ulaw while other channels are using gsm ?

 tia.