Jeremy,
There can be only [1] Prime DN, but you can have up to 8 devices associated with it and [2] of those can be softphones. The way Mitel suggests setting up extra phones is the following.
Primary: 1000
Secondary: 1*000
Tertiary: 10*00
Quaternary: 100*0
Quinary: 1000*
After that I am not sure how they would progress, but I have also seen *1000 for some phones in previous databases. Once they are created they would all go into a Multi-Device User Group or Personal Ring Group Depending on licensing I believe.
In the MiCollab just go under the Phones tab and add a Secondary and Tertiary phones and then make sure they are in a Ring Group together so if they dial the Primary DN it will ring any of the phones that are not set to Absent.
Thanks,
TE