Have you looked at your activation-service.log. Also try going into the phone proxy and manually setting the user to active. Then have them plug the phone in and issue the show user command. You will see the counter increase waiting for the phone to register. I have had this with several of our phones on the proxy. You wil see the phone register and deregister just as fast. In the cases where I had this happen we had to replace the phone. Have you done any updates to your call manager or put new updates our for your phones. The times that this happened to us the phone downloaded the updates but could never finish.
