What is the meaning of that tag? The documentation says "Primary Device for mobility users" which has me scratching my head. When I look at my fully configured users with RDP and mobile Jabber devices (and primary device filled out on the device), I still don't get a primaryDevice tag (in fact the tag isn't even in the getUser response unlike your usual "if there's nothing, the get operation returns an empty tag)
I don't know why it's not returned with getUser. I can check with the engineers, but it could be an oversight, or there may be some other means of getting the data. If I understand it correctly, the primary device exists for licensing issues. When you set a primary device, it is recorded only once in the number of phone licenses so you can assign the phone again without using up another license. I'm not totally familiar with license issues, though.