Its weird with calling groups and transfers from Voice Mail. If the "Transfer Return Extension" of the Voice Mail ports is properly programmed in #306 as Ext. 10, calls from the Auto Attendant to a caling group will ring forever in the calling group.
If the transfer return extension is set to itself for each VM port, the call will ring X number of times at the calling group, then return to the Voice Mail port that transferred them. BUT, it will return without any "mode codes" to tell the voice mail how to treat the call, so after a pause the VM will say "Transferring to Operator (or receptionist, whatever)" and send the call to the Call Answer Service Operator, which is Ext. 10 on Partner Mail VS, or can be redefined on Partner Mail and Partner Messaging.
The gotcha is call volume. If you only have a 2-port mail, and lots of calls, if is possible that a transfer return from the calling group might hit a busy port.
And all this does is get the call to the Operator to handle, instead of cover to a voice mail (unless you let it cover to the operator's VM)