asterisk users - Jun 2005 - How to dimension Asterisk - that is used solely as callback server - only sending untranscoded voice between two ISDN channels on PRI ?

Hi,

I wonder how I could dimension Asterisk system that will be used solely as 
callback server :

- when user calls it registers ring, hangup and calls back - it gives him a 
dial signal and calls dialed number on another ISDN channel out....

that means plain transfer between two ISDN channels - no transcoding or any 
other stuff...

I guess using Asterisk in this way I could dimension for higher number of 
parallel calls - but how many ?   General rule is to put 1 octo PRI card per 
PC, but could I add another one or more if used in described way ?

Any similar examples of dimensioning ? Do I get any better with cluster for 
such purpose ? Any other advice ?

Thanks in advance,

regards,

Rob.