Help With Ring Groups and Forwarding

Hi All,

I have a client that uses a ring group to forward all incoming calls to the four extensions in the office. However, they need a way to forward calls from the main inbound route to a cellphone number of a various team member, depending on which team member leaves the office etc. so the number that the extensions will be forward to will always be changing.

How will I be able to allow the client to forward all the extensions to a particular external, PSTN number?

Any advice would be phenom!!