Resetting a 7406 Cordless Phone

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I have a customer that purchased (2) 7406 telephones and (1) base unit. I was able to install the first handset without any trouble but the second handset is not accepting the default password of '000000' when trying to change the handset ID. I tried the more common passwords that people might change them to such as '123456' and '111111' and so on but no luck.

Does anyone know of a way of resetting the handset back to the default setting of '000000' without sending it back to the supplier?

Any suggestions would be greatly appreciated.