Do you have a MiCollab? I've seen a similar error and assume you do. Here is how I would approach it with an example:
User1 is ext 1000 which is a full user on MiVB and MiCollab and the extension pinned to the phone.
User2 is ext 1001 which is a trusted extension (can be fully licensed but doesn't need to be) on MiVB and full user on MiCollab
Put User2 as a multi-call on User1's phone so they can get calls and then an MWI key for their mailbox.
I'm not aware of MiCollab so I'm going to say no. This is an inherited system so I'm still getting accustomed to it.
Trying to add user2's extension to the phone as the second line results in "The value entered for the following field already exists as a key : 'Hosted User Service Guid' "
Is there any reason you're not having them hotdesk?
My understanding of that is they would sign into the phone and the phone would then be associated with their extension. I need multiple people to use it during the same shift, so either could answer at any time. Also this is the way it was previously setup and I'm just trying to add user2 to the mix and it's not working.
What voice mail are you using? Embedded, MiCollab/NuPoint, or something else?
Whatever the default voicemail is in the admin page. I'm creating the vmail box and assigning it an extension then I'm trying to add that extension as "Message waiting Indicator" key for that extension.