The DOT button not working on the Mark I Beocom 6000 is one of the most common problems you will see with this version. As Neil and others have noted you can purchase replacement keypads from your dealer. However, for just a few pounds more you can also purchase a upgrade kit which will provide the software and functionality of a Mark II, but still charge and work with your Mark I base.
It should be noted that if a fault occurs with the Mark I PCB (normally teh RF chip fails) then there are no replacements available, so you only have the option outlined above.
You can purchase an upgrade kit part number 3455103 which consists of a new board and display, new front panel and two connecting pieces. Most dealers do not know about these kits, hence why you will need the part number. In fact everything you can see below is in the kit plus an speaker not shown:-
You will also need a tool to open the unit, but with considerable care it can be opened with other tools. There are NO instructions with the kit except for a message to say that all parts must be used. The reason it says this is because some people may not used the two items in the plastic bags which look like a piece of cut off black and white plastic, but they are actually made of rubber with metal conductors inside and MUST replace the existing ones.
The upgraded unit will still work with your triangle base station/charger provided the software is at at least version 1.9