If you know the admin password for your Yealink IP phone go to Menu > Settings > Advanced Settings > Reset
If you’ve forgotten or never received your admin password?
Press and hold the OK button it could take 6-30 seconds depending on the model. My SIP T43U took about 6 seconds. It then asked Reset to factory settings? Cancel/OK. Hit OK.
The factory default username and password is admin.
Some telco’s act as if VoIP phones are Customer Premises Equipment and the hardware belongs to them. As such they upload custom firmware with different default usernames and passwords. Tough luck if you need service or support once the equipment is out of contract. Your Telco should provide you with the Username and Password if the equipment now belongs to you or they don’t want it back.
There may be a way to upload replacement Yealink firmware via USB, but I haven’t found a way yet. Comment below if you’ve made this work.