[cisco-voip] Rewriting incoming caller ID using CallManager / router?

[Robert Kulagowski]

Is there some way to rewrite incoming caller ID information using just 
callmanager / routers?  We have a 6-digit dial plan, and I'd like for 
calls coming from one of our offices to be reformatted so that it's 
presented to the caller as sitecode+XXXX (rather than 10 digits)

The inbound voice gateways are MGCP, and I don't believe I can use 
translation patterns and rules on the voice ports to manipulate the digits.

Any thoughts?