4.50 saw the addition of a new field in MMC210 called FWD HOLD CLI (May have changed names since I last touched it). When off the system will use the office number. When on it will send the CID of the incoming caller as long as the provider allows it.

Pre-4.50 and on anything 500 or older retransmitting the inbound is the default (and fixed) behavior. The only real requisite is having CID enabled for the inbound trunks and allowing trunk-to-trunk transfers.