Hi, Have you checked network part? Is there any difference between working and non working phones in terms of the way they communicate to CME over switched network?
If network is good then browse all the option under setting button of working and non...
This document could be helpful to start investigation
http://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/118744-technote-cucm-00.html
7900 phones use tzupdater.jar file whereas 69xx phones use ...
