A couple of things, create an intercom key on the customers phone, feature code 645. This creates a second pathway and the phone is not technicality busy unless two calls are happening at once.

The did number could be forwarded as opposed to the extension itself.
Forwarding is programmed in two places, preset in flash 50/51 for the phone and in feature 43 for the did number. The latter is not usually used unless the number is ringing on multiple phones at the same time, like an operator and a backup.

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"Old phone guys never die, they just get locked in some closet with an old phone system and forgotten about"

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