asterisk users - Mar 2006 - Passing Digits between ISDN PBX and Asterisk

Hi All

I have an Asterisk box using a Sirrix card sitting between our PSTN and 
an ISDN pbx.  Calls from the PSTN are forwarded to the PBX ok.
Calls from the PBX are having problems - the digits being passed are 
being "garbled".  The numbers from the PBX are totally incorrect and 
sometimes too long or too short.

Anyone know what could be causing this?  I would like to find some more 
info on the ISDN layers and protocols, but I haven't found a good source 
on this.

Thanks
Garth