I would think that a full 23 Channel PRI would be suffice unless they do alot of off-net transferring. I'm guessing that the 21 Centrex numbers are in one location (again guessing) so if they are all on the same system the system would be handling the transfer not centrex but if they are sending these to other numbers in other locations, unless of coarse you are putting remote IP phones, this would take 2 trunks(channels) to transfer the call out.

Hope that helped ya!

Gary