Caller ID Pass thru on Forwarded and Diverted Calls
When calls are made to External Numbers, the default Caller ID of the account is used. You can specify the caller ID for External calls in Ring Groups or Call Queues only.
Route the incoming to a Ring Group with the external number as an Agent. Select Caller ID Pass Thru and press save - now any calls to the ring group will call the external number and the original caller id will be forwarded.