Gerd,
You are correct that both Mark I & Mark II Beocom 6000's can be updated with new firmware and there is different firmware for each country.
Below is the UK command file which confirms this fact.
[SwPackSection]
ConfigFile=bc6000hs.cfg
HexFile=06101315.hsf
LoadEepromFile=BC6000H_GB25_1.swp
Info=BeoCom 6000 handset software version 2.5 for United Kingdom, country version 25_1.
hwver=9810
However, to have the handsets firmware updated it would have to be taken to a dealer to connect an adapter to the handset for that update to take place. Given it is a Mark I handset, it is only a small cost to upgrade it to a Mark II which automatically comes with the latest firmware.