The issue is that you really need to have the extensions subscribe to a common mail box. Using the concurrent registrations (not withstanding the limits mentioned) would cause all the phones with the same extension number to all ring at the same time.
This is why most systems use a group mail system that forwards as the phones are typically only able to subscribe to extensions that are set on the device itself and the MWI is a function of the NOTIFY to the subscription when a VM is received.
I do know of one system that allows for BLF monitoring of an extension’s VM box. Whether or not the UCM will do this is an unknown and I can’t test it at the moment, but the other system of which I am referring to is Asterisk based. Here is a snip from the manual -
*98 Remote Voicemail, remote entry to the voicemail system to listen to voicemail for any extension. Requires the user to input both an extension number and the voicemail password for that extension. Can also be used as a BLF to monitor and access a shared mailbox. To create a BLF to monitor the shared mailbox 2001, you should create a BLF containing *982001.
I guess you could try it and see.
I had to take my UCM that was setup out of service as a client’s IP-Office system just failed and I needed to get him up on something quickly as they are a health care facility and I already have 3 of their other offices.