Hello,
I also have this deal from Three (different handset, but same 24 month contract - unlimited data, 350 flexiunits etc).
The deal is good as long as you don't go off the Three network and manage your voicemail by either using the above referenced service for voicemail, or disconnect it.
The customer service for Three is however, one of the worst I have ever in my life had to deal with ... perhaps Dell compares, but very few others. I suspect this reason and this reason alone will see both myself and many others leave Three in favour of another mobile phone provider once contracts come to an end.