I tried "Dot 70008" and the message below was shown on the Beocom 6000 screen:   PHONE BOOK ** NOT BASE 1 **   Then I tried to register the Beocom 5 on the base but with the same result "Searching base Wait..."  

I forgot to say that after pressing Dot 7 000 8 you should press OK (centre of wheel). Does that make any difference?

If I press "Dot 7 000 8" then after 4 seconds it goes to the main screen where the current time and date are displayed. If I press "Dot 7 000 8 Ok" then it goes to the main screen where the current time and date are displayed. Pressing Ok at the end just makes it directly jump to the main screen where the current time and date are displayed instead of waiting for the 4 secs timeout. I'm sure the Beocom 6000 is connected to the Beoline base since I can make phone calls successfully.  

I just tried the option "REMOVE HANDSET" on the Beocom 6000 and I get the same message on the screen: PHONE BOOK ** NOT BASE 1 ** According to the manual this message means "The Phonebook is stored in base 1, but base 1 is inaccessible." It is really weird since the Beocom 6000 still can make phone calls so the base 1 must be accessible being the only base registered on the phone. In summary it is not an specific problem with the command to reset the base since this other command fails as well.

I presume that the BC6000 was originally connected to another base that you own, known to it as 'Base 1' (with your phonebook stored). If this is the case, your phone still has registration to 'Base 1' stored - even though it is currently connected to a different base (Beoline 2) to make calls. Perhaps the BC6000 can only reset the first base that it is connected to, and this is not currently the Beoline. You could try resetting the BC6000 handset: Dot 7 000 9 OK. Then re-register the BC6000 to the Beoline 2 and try the Dot 7 000 8 OK trick to reset the Beoline 2. However, I do know that there are software incompatibilities between older handsets and newer bases, so perhaps you simply cannot reset a Beoline 2 from a Mk1 BC6000. I can't think of any other suggestions, apart from disconnecting everything (perhaps even removing phone batteries), leaving it overnight and trying again tomorrow. Beocom/Beoline registration can be quite temperamental and sometimes it takes a bit of patience and luck!

I tried everything you suggested but nothing worked. I'm going to try to return the phone to the seller. Thank you!