asterisk users - Apr 2004 - Newbie Question: ISDN and Capacity Planning

Hi all;

I am planning a PBX/Voice mail system for a small business (approx 12 
employees with phones).  They have an inbound ISDN PRI, which is 
probably irrelevant because all inbound calls are routed first to 
receptionists which rarely route the calls on (client is a medical clinic).

Any idea what sort of capacity planning I should be looking at?  Any 
minimum and optimal figures would be good, as my bid will include 
redundant systems.  Any hidden gotchas with ISDN I should be familiar with?

It should be noted that phone use among most of the staff is low to 
average-- calls are rarely routed back to the doctors/nurses but they 
have occasional need to call out.

Best Wishes,
Chris Travers
Metatron Technology Consulting