You should use only Sanyo Ni-MH batteries model number 3HR-AAAU as replacements in your Beocom 6000. which are available from your B&O dealer.
You will see a lot of so called replacement batteries which look the same, but are not. The key difference is their MaH (Milli-amp hour) rating which because the Beocom 6000 power adapter is not a regulated power supply means will often not charge correctly, therefore the phone does not work more often than not.