There is a special process for rebooting your phone. In fact, for the Yealink T5xx phones, there are a couple of ways, so let's check it out!

Rebooting/Restarting the phone
T54W
- Long press Cancel (picture of an X) on the keypad when phone is idle
- When option to reboot comes up, select "yes" or whatever action means "yes"
- Phone will reboot as normal
T57W
- From the idle screen select Menu then move to Basic --> Reboot
- Select Reboot
Alternative reboot methods
T57 and T58
Simply press and hold the * (asterisk) button on the phone until it asks you to reboot.
T57W
- From the idle screen, select MENU
- Navigate to BASIC
- Select REBOOT
T58A
- Swipe left or right on screen to get to second idle screen (or swipe down from the top to access the Quick Settings menu
- Select SETTINGS
- Navigate to BASIC SETTINGS > REBOOT. Select REBOOT
And that's about it! Once done, you should have a fully rebooted phone!
Time for a new headset?
Many user of Yealink phones have switched to the Leitner LH370