asterisk users - Feb 2012 - Set T38 protocol

Hello,

I'm trying to send a fax with sendafax aplication and receive the fax with 
the receiveFax aplication on the same Asterisk Server (1.8..8.2).

All work fine but the PBX always use T30 protocol.

Is thes a variable or setting to configure Asterisk to send and receive this 
fax with T38 protocol only?

Thank you

Regards