checkout this link also.
http://www.voip-info.org/wiki/view/CallManager+ExpressBTW, the easiest way to do this is to assign different outside access number to the differrent PRI. Example you may have a dialplan 9T that access PRI1 and 8T that access PRI2. This way people in company1 know to get an outside line dial 9 and company2 know to get an outside line dial 8.