Important difference on R6 and R7 ACS with Partner Messaging - set the Transfer Return Extension setting for each voice mail port to Ext. 10. Calls to extensions without voice mail coverage (or with VM Cover turned OFF) will ring at the extension and then go to Ext. 10 when not answered.
If you don't change those transfer return extensions, they will return to the voice mail, and it will KNOW which extension it is returning from, and if it finds a matching mailbox, that's what will answer (just like VM Cover was ON)