Depending on your controller's software level and how you have this setup now, I would do it one of three ways:
1) With Ring Group. I would specify that the main number rings a ring group, composed of all 5 AA's phones. That way you can set the time-out and overflow point as the voicemail box for the main number. (Allows to multiple main number answer points
2) If the main number is setup on AA1's phone as a separate single line, then place multicall keys of the same dn on the other 4 AA's phones. (Lets each AA answer one incoming call, and to see the difference between internal and external calls.)
3) Set the main number to ring a hunt group. Create a series of key system keys on each AA's phone. Place those key-systems into the main number's hunt group. Then using call rerouting, send any unanswered calls to voicemail. (This allows for each AA to potentially answer multiple incoming calls, see the difference between external/internal calls, and for them to have access to held calls that a different AA might have placed into that state.)