Call Pickup from Ring Group at another phone.

I have a customer who has their reception phone programmed with two other phones to ring simultaneously in a ring group (those phones are the backup receptionist and lunch room). During the day there might not be anyone at these three phones so they would like to have another extension have the capability to pickup calls to their main number from time to time. They do not want this 4th phone to be part of the ring group so it rings for every incoming call to the main number.

In the PBX world this was called Call Pickup or Directed Call Pickup.

My customer is using Yealink T48S phones on their RC account. Has anyone figured out a way for this to be accomplished?