Epicor Web Services Customer Lookup

10.2.600
I need to do a lookup using the Web services, but everything I see in the BO description uses the CustNum for lookup. Users have no idea what the CustNum is and use the CustID for all their interactions. Maybe I am just not getting this, but it seems like the design of the CustomerSvc BO is requiring me to get the CustNum with a BAQ before I can use it.