The extension numbers for the 4 calling groups are 71 for group 1, 72 for group 2, 73 for group 3, and 74 for group 4.

Use #502-{1/2/3/4}-{Ext No.}-1 to assign extensions to groups (Group 1 is typically used for All Page, but it may not apply to your set up)

Make your AA selector code transfer to Ext, 7X

What do you want to happen if no one in the group answers the call? Transfer Return Rings #105 can be set from 0 to 9 (0 being no return, just keep ringing)

Since you can't assign a mailbox to any of the Group extensions, set #105 to a non-zero value, set #306-78-78 and #306-79-79, a call transferred to the group will return after X rings, be picked up by the voicemail, which can't process the call, and then be transferred to Ext. 10 (the system operator)