the only way you will get out is if they change a condition/term of the contract such as capping Internet use ( which most of them have announced recently!) and then you have teh right to cancel at no cost.
Otherwise you can try calling them and seeing what you can negotiate after explaining your situation and why your circumstances have chnaged but I wouldnt hold your breath.
If it simply that you thought you could afford it but now realise you cant then they have you overa barrel, if you have lost your job or something that clearly affects your ability to pay then you may have a chance.you could try and get it dropped to the cheapest tarriff going
From their perspective they gave you a phone worth probably a couple of hundred quid( or even more depending on model, my n900 are ~ 500 quid sim free!) which s/h is now worth a fraction of that. they do this because they have you nailed for the next 24 months as regular income and calls/texts on top
