The default PIN code is 0000 so give that a try.

Good luck!

Of course I tried with default 0000 code but it doesn't work. Well probably all will work well, but I was so stupid to unregister the base station without knowing the pin code. So the pin code was predefined in the handset.

I'm sure it's impossible for anyone here to help you. I'm sure your dealer can help you though.

And if not I surely can help you, have a whole box of those :-)

As you say you bought this second hand I assume that you only have one handset and base?

With the early version of base it was possible to register up to six handsets to create a network of phones. If you registered six handsets and tried to register a seventh you were prompted for a pin code.

I have come across instances where customers phones have lost contact with the base and instead of switching handset off and on again, they have re-registered the same handset to the base. This creates a "phantom" handset registration and uses one of the six available slots. If this is repeated, you can end up with one base and handset but the base thinks it has six handsets registered. In this instance it is possible to delete one or more phantom handsets but this requires access to a handset that is registered to the same base and is working.

If you only have one handset I'm afraid I can't offer any further suggestions.

Opman

If you add more than 6 handsets to the old pyramid charger,or 8 handsets to the newer Beoline base it asks for a pin number. 
Just remove all handsets except the one you are using, then the PIN problem should be gone. 

Maybe previous owners have handsets registered.

Hope this helps

Sorry, just read Opman's post. 

0000 never works. I have the same problem with a first hand phone. Switch bases and still asking for pin code. 0000 won’t work. My 4 digit PIN number that I always use, also, doesn’t work. Just in case I set my numbers long time ago. I just change the battery from the hand set, set the 9#9#9*355 or something like that to reset battery new. Base started to stop working, then work, then not working. I registered again and now can’t connect again because always ask for pin code. And no matter what code a write. Always error. How can I reset phone and both bases. The pyramid and also the PSTN black box. I have a few of those. Even a new one that also ask me for pin code. I read in other forum that might be something with the phone plate???