Ktlcom,
There is no physical reset button or similar in the base of the Beocom 6000.
Your problem is not going to be to do with the pin code, but more likely is that six handsets have been previously registered to the base and not un-registered. Due to a software error, if you try and register the seventh handset you will get asked for a pin code. No matter if the pin code is correct you cannot register the handset.
If you do not have access to the handsets which were previously registered to the base so you can reset the base or un-register the handset, then you will need to get your dealer to connect the service jig to the base.