I have a new SL2100 running on analogue lines, if any extensions presses the VM button on the handset, it just rings out. The voicemail system doesn't answer the call.

Does anyone have any ideas why this may be happening?

Did you change the numbering plan from default? VM works out of the box, but if you made changes to extension numbers + department group 50 (VM/AA) then it may not work. Defualt VM pilot it 3999. If you did change extension numbers then you need to do the same for the VM box numbers.