You've probably bought a new phone by now but I had the same problem.
First I reset the main handset by holding down the R button (bottom left) until the phone turns off. Remove the batteries. When putting the batteries back in hold down the * button. Default should come up, press tick and enter the pin. On the base hold the page button until it beeps and the handset should register. Then go into the menu - setup - pin code. Change the pin and it should work fine.
Use the "Reset if you have forgotten your PIN code" instructions.
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