have to agree: the phone companies do not just "give" things away, and if there's one thing you can guarantee is that they will not be out of pocket on the deal. IMHO 24 months is a very long time to be tied to a £35 contract - do you know what your work situation will be for sure? What would happen if your car/boiler broke down and you needed some extra money? It's never good to be just at your limit with spending. You could have £5k a month, but if you have outgoings of 5k, then the slightest little unexpected thing can push you over.
Do you actually spend 15 hrs on the phone each month?