there is a very easy way of doing this:
1. locate the voip dial-peer that leads to that specific customer
2. add the command "clid network 12345" under that dial-peer.
this way, every time this voip dial-peer is used to route the call to that customer, the caller ID displayed will be 12345 (which u can change "12345" to anything you want).
You can do the same thing under the Pots dial-peer too, if you want that caller ID of any call originating from the PSTN to be changed to 12345 at all customers location.this latter solution is more general, the former is specific and granular. and i have been able to accomplish this on 2600xm, but i think it will work on the AS class as well.
Hope it helps
Eyabane