I was at a clients site today activating a new switch for printers and other "low speed" devices. While I was there they had an interesting issue with the Cisco IP phones. I'm not their "Cisco Phone Guy" so this was not a system I configured. It's actually the first Cisco VOIP equipment I've touched in years.

The issue was, randomly the IP phones would change their extension number and what user was logged in. I actually watched one phone for about 10 minutes. It would register as one extension, then 30 seconds later re register as someone else.

I managed to log into their Call Manager System ( both GUI and telnet) it APPEARS that each phone MAC is set for only one extension #. So I was a little confused on the issue.

Eventually I gave the 2581 ISR a reload and everything came back up as it should.

Has anyone seen this issue before? I can PM a running config if you have any insight.

As I said, not my phone installation. I'm just interested in the solutions.

The only thing I can think of is extension mobility? what version of CM?
"I managed to log into their CM System?"

by telnet?
First yes, only One Mac per phone, you can not have duplicates. Also I don't know what a 2581 ISR is but is it a router? Why would reloading that fix the call manager or even have anything to do with the phone configs? just my questions.