Assuming it's a BeoCom 6000 Mk2, on changing the battery you should press: 9#9#9✱655 OK

Guy I entered the code but there is no difference. Maybe it is not a Mk2, serial number is 15734840 and it is from 1997. Is there another method-solution ?   Thanks

Sorry but that's the only battery reset procedure that I know - I think it may work for both Mk1 and Mk2 models. You could also try resetting the handset - make a note of any stored numbers first.  You may also want to be sure that your PSTN base does not have a pincode set. If it's a Mk1 (dot button to the left of the circular wheel), press (dot) then 70009 If it's a Mk2 (INT button to the left of the circular wheel), enter the service menu by pressing:  9#9#9✱INT then scroll to 'reset handset' and press centre button. You will have to re-register with the PSTN base after a handset reset, which may need a pincode as mentioned above.  Default is 0000.