To reboot a Cisco 7940 / 7960 serial VOIP handset without unplugging the power brick (or the ethernet port when using PoE / Power Over Ethernet), you can press the following three button combination:
[*] +  + [Settings]
The “Headset” button light will light up briefly and then turn off, then the same thing will occur with the “Mute” button light, and then finally the same will occur with the “Speaker” button light. The phone will then reboot.
This can be very handy when testing various configuration changes, and constantly need to reboot the phone to refresh the settings.