In essence, you entered into another contract, a further term of 18 months in consideration for a new phone.
I take it they advised you that you would have to enter into a new 18 month contract in order to get the new phone. Did they just send you one or did you have to go into a branch to pick and/or collect one?
You have carried on with it for 7 months without a problem, it's only now because you want to get out of it you have raised an issue.
The amount required to get out of the contract will, of course, reduce as time goes by. Depending on the minimum amount you are contracted to pay monthly x the remaining 11 months you should be able to guage when you will be able to afford to pay off the outstanding amount.