I'm assuming the user has a greeting recorded for greeting #2 that offers the caller the option of pressing 4 to be transferred to cell phone. If this is the case the user may not even know they need to have a Greeting #1 recorded that does not offer the option. Having a greeting recorded in each location creates the On/Off capability.
When G-1 is used forwarding is off...
When G-2 is used forwarding is on...

End user can also switch between G-1 & G-2 remotely.

If forward to cell is offered in AA main as one of the option branches the that is different set of circumstances.

John

John