Customizing MWI Locations

I employ common voicemailboxes created as extensions with no end-point device. I need to have MWI on one of the extensions on a Grandstream phone.
You used to be able to change the xxxx@device parameter to YYYY@device, where “YYYY” is the VM for the common mailbox extension. This would show the presence of a message in mailbox YYYY.
This no longer works. Is there a new approach?