Question:  I bought a used DX-100 system and I'm registering some new beltpacs to the base station.  I can only register 9 beltpacs before the base station refuses to register anymore beltpacs.  What am I doing wrong?
Answer:  You are correct in that the MB-100 base station is capable of registering up to 15 BP200/WH200 communicators in the base stations memory.  The first time you operate the DX100 system, you must register each BP200 or WH200 to the base station for use with a specific base station.
The base station will then recognize all registered Beltpacs/ Headsets/Speaker Stations when their power is on, and will know the difference between them and other equipment operating on the same frequencies. If a Beltpac/Headset is added, replaced or repaired later the new must be registered but the old ones remain in memory. Since only 15 Beltpacs or Headsets can be in memory, whether currently in use or not, all memory must be cleared to remove any old Beltpac/Headset registrations. 
So clear the memory on the base station to remove the beltpacs/Headsets no longer in use and then register the currently used beltpacs and headsets.