Интсрукция по настройке TGP500_09_AG-Russian.pdf
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For Panasonic KX-TGP5xx users: factory reset
The Panasonic manual and web interface for the KX-TGP5xx phones doesn't say anything about a complete factory reset procedure. The only procedure given is for the web settings which doesn't clear everything.
In case anyone is interested, there's a username "customerserviceid" with password "cspass" which gives access to a different web interface where you can find the option for a complete factory reset in the Maintenance menu.