The only way we’re able to change the Caller ID for calls forwarded from numbers is to forward them to a different DID on the PBX – there’s no way to differentiate between calls coming into one number.
What you can do, however, is forward calls to an otherwise unused DID (such as a DID on their account set to deny access), replace the Caller ID for this DID, then point that DID at your main queue.
Take the below scenario:
Calls are forwarded from 02083461132 to 02035887912 (currently set as Deny Access).
02035887912 is set to replace Caller ID to something along the lines of 'Old System %CALLERID%'.
02035887912 is then pointed at your main Queue.
The end result of this would be that calls from the numbers forwarded into our system would reach the end extension with a different caller ID to other calls.