According to the manual, you're supposed to set Transfer Return Extension for each of the VM ports to Ext. 10. This is so when a call is transferred to an extension without Auto VMS Coverage, and goes unanswered, it will transfer to Ext. 10.
This is not the case for Calling Groups, the call will ring forever.
BUT, set Transfer Return Extension for each VM port to themselves (actually factory setting) - #306-XX-XX (VM ports are 18 and 19, set #306-18-18 and #306-19-19
In Partner Messaging, create mailboxes, WITH a phone, for the Calling Group you are going to transfer to (71, 72, 73, and/or 74). While creating the Group Mailbox, also assign it as a Phantom Mailbox, with Mailbox 10 as the destination.
Record and activate a greeting in the Group Mailbox (can be the same verbiage as Ext. 10's mailbox).
A call transferred from the Auto Attendant to a Calling Group will ring in the group until the Transfer Return Rings in #105 is met. It will then RETURN to the VM port that originally transferred it. The system will see a matching mailbox, play the Greeting, and deposit the incoming message in Mailbox 10.
If your Partner Messaging is R7, you can have multiple Phantom mailboxes send their messages to Mailbox 10. If you have Release 6 and have multiple Group mailboxes, make the phantom destination an unused AA's General Mailbox, and set Ext. 10 as the MW receiver. So say a call transfers to Group 72, comes back, Mailbox 72 answers, the incoming message is placed in Mailbox 9992, and Ext. 10 gets the light.