T20 bought at second hand store. Unable to reset it.

[Lucia_Yealink]

I can show you a solution to change the password without resetting the phone, steps as below:
1. I attached a .cfg file for you, which has a simple parameter:
static.security.user_password = admin:admin
2. put this file on your provision server(in my example, I use HFS as my local server)
https://ticket.yealink.com/BugFile/image..._84031.jpg
3. long press a line key, and choose the Type as key Event, Key Type as Zero Touch:

4. after this linekey is set, press it once, you will see below screen, and then press OK-->next


5. when you see below screen, input the URL manually-->press OK


I know this is not a simple solution, but please note that, customer just need to set this line key once, if he meets same issue again, he don't need to input the URL again.

Please have a test on your side, and let me know if any misunderstanding.