I have changed the time zone but all my IP Phone always the old time.
I want to modify the time and put a specific time for all my IP Phone, how to do?
This is not an uncommon issue that should be quite fixable :)
Troubleshooting Incorrect Time Stamp on IP Phones in Cisco Unified CME
After changing the time-zone setting you could follow these steps.
This command works with the vendorConfig section of the Sep*.conf.xml configuration file, which is read by the phone firmware when the Cisco IP Phone is booted up. Certain phones, such as the Cisco Unified IP Phone 7906 , 7911, 7931, 7941, 7942, 7945, 7961, 7962, 7965, 7970 , 7971, and 7975, obtain Coordinated Universal Time (UTC) from the clock of the Cisco router. To display the correct local time, the time display on these phones must be offset by using this command.
This command is not required for Cisco Unified IP Phone 7902G, 7905G, 7912G, 7920, 7921, 7935, 7936, 7940, 7960, or 7985G.
For changes to the time-zone settings take effect, the Sep*.conf.xml file must be updated by using the create cnf-files command and the Cisco IP phones must rebooted by using the reset command.
under the telephony-service -
no create cnf-files
Then reboot the individual phones :)
This was really covered nicely by Paul in this thread :)
Hope this helps!