Funnily enough I was looking at a phone yesterday and saw a lot of these offers. I did a bit more digging and found that with all the companies offering money back, they were all with out exception, new start businesses limited by guarantee.
To my mind, I guess that they will say that you can get your money back in 4 months time, (and wait the prescribed 45 days after that) but that in that period they would have disappeared with their money. Of course, your contract is with a main service provider, so you can't cancel your contract.