asterisk users - May 2005 - ISDN RAS and data calls

It seems like when I use PPPD-command, or ZapRAS, Asterisk doesn't make 
it a "data" call, but a regular voice-call.
My ISP change their behaivour depending on the incoming call-type (data 
or voice).
If it's voice, they try to open up a V.90 connection. Else (data call) 
it will reply with PPP directly.
The both methods uses the same modem pool number.

How can I tell Asterisk to inititate a data-call instead of voice-call?
I'm using .call-files to connect to the ISP.
--
Daniel