How to cancel my Orange contract with unpaid bills?

I went to America in January this year. When I returned, there was a huge data roaming charge on my phone bill, totally nearly £200, even though prior to my trip, I purchased a data-roaming package
for £25.

Orange claimed that I used 25MB (beyond the allotted 30MB), at £8 per megabyte.

I dispute this fact. I paid £100 but would prefer not to pay the rest.

The Orange customer care is very unhelpful. They've terminated my service due to unpaid bill.

However, my charges seems to continue accruing.

I'd like to terminate my relationship with Orange and go to another company.

However, Orange claims that I have to pay all the outstanding bills, and also £75 to terminate the contract.

I think this is blackmail.

Also complicating the situation is that my phone (an iPhone) is bought through Orange. I believe it's locked, so I can't go to another company.

Basically I'm stuck here:
(1) My phone doesn't work
(2) I'm being charged for service not provided
(3) I can't go to another phone company

What can I do in this situation?

I think I can get the phone unlocked in some shop. Then I can sign up with another phone company.

But then, will Orange charge me for unprovided service till eternity?

Take the emotion out of it if you can - words like 'blackmail' do nothing except wind everyone up.

What exactly are you disputing: the package you bought or the amount of data for which you've been charged? Both should be capable of being checked. The fact that you have paid another £100 weakens your case, because it does rather suggest you know you exceeded your data limit.

Read your contract or (if you'd prefer not to wade through a mass of small print) ask Orange to point out the clause which requires you to pay £75. Unfortunately there probably is such a clause - you will have bought the handset at a cheap price, or got it free, because Orange expected you to use their services for the duration of the original contract. There's also likely to be a clause saying you can't terminate the agreement early if bills are unpaid.

You can cancel anytime you want… Your bills however… Hang around until you pay them off…

You will have to pay them what you owe them legally. They have justified the charges and if you want them to continue charging you interest and other fees you can wait it out.

The only other solution is to buy a Pay As you Go sim card but then your debt with Orange will still remain due and they will continue charging you interest and late fees. Its your choice now.