Could you please share more details? Which use cases do you want to apply?

We have Group Call Pickup and Queue Call Pickup features. Probably some of them will be applicable for you.

Group Call Pickup. This particular user is not in this specific group but we need them to pick up the call when this group is not available to pick it up. It doesn't ring on this phone but he can hear it and knows which group it is.

Same. Please

Probably Queue Call Pickup feature (QCP) is that you need.
Users are in queue. Somebody call to queue and one of available agents can pick up call. If nobody answers, QCP extension on the HUD allows an user who registered as pick up person (he can belong or not belong to the queue) pick up this call.

thank you