Perhaps an ideal place for something like this would be a Line Status page that replaces the "Devices" page. That page could have a layout that includes the following information:

Phone number:
Device IP: (if you easily see the public IP with your current PAP2 provisioned configuration--kinda useless if it reports a private IP)
Device Status: (Not Registered, Idle, Active call(s) with [peer#(s)] )
Last Registration: (could be useful if current status is "Not Registered")
Re-registration: (better wording than "Register Expires", optional if you think confusing)
Registration Server:

My primary concern with this type of page is user confusion. This potential for confusion simply needs to be balanced with perceived advantage, such as for troubleshooting.

FWIW, with CallVantage, we had none of this information, and all I really knew (or cared) was that both incoming and outgoing calls worked. Ideally, any switching between servers is transparent (and thus unimportant) to the end user.