To reset Yealink T48G to default you will need to Follow steps below
1) Please long press the "ok" button
2) Then there will show a warning sentence,select "ok" then the phone will reset,
3) After that,please try to use double "admin" to access the web UI!
If it can not work either,please contact the server provider or phone provider for help,because if you have a DHCP or PNP
environment,once you reset the phone,phone will send request to the server,and download the cfg files again.
So if there is sentence about password and username in the cfg file,please delete it,then test again!
Any question,please contact MiZAZA Team