I do not belive that an upgrade will affect this problem, nor do I believe the problem is phone load related.
The Extension Mobility application does a local authentication of users before logging them in. Since you did not mention it, I am assuming that we are not working with an Active Directory or Netscape/iPlanet Directory integration. I am also assuming a standalone CRA server performing your Extension Mobility services.
If the problem is persistent for some users, but not others, I would check the DC Directory on the CRA server. If you are updating your users on the publihser, but the replication between the two DC Directories is not working, then you will get this error when the CRA server is attempting local user authentication.
Thanks,
Dan Keller
Cisco Systems