To reboot Cisco 7940/7960 IP phone hold down the following keys :-
"*" and "6" and "Settings"
you have to turn on telnet on the phone using SIPDefault.cnf entery:
# Telnet Level (enable or disable the ability to telnet into the phone)
telnet_level: 2 ; 0-Disabled (default), 1-Enabled, 2-Privileged
then you will be able to reboot it remotely.