Please help.

I'm new to this forum, after looking for advice on how I go about getting out of my 3 mobile contract. For the past 3 month, I have received bills of over 120 pounds (122, 145 &170) when normally I would only be paying around 50 pound. I have 750minutes talk time with unlimited texts which is 38+vat. For some reason my talk time was alot more than normal, where I blamed a problem I was having; hence the looking into cancelling contract.

What was happening was even though I had a signal on my phone, when making a call, before it connected it was saying call Ended 0:00. I check my recent bill and realised I had alot of 1 minute phone calls and thought that i was being charged a minute talktime everytime I made a call under a minute, even though it wasn't connecting and the reason I had a lot of them was at times I had to try 10 times just to get through. I argued the case as I couldn't think of anything else as I knew I hadn't used my phone any more than normal. I also assumed that I had always been charged a minute for a call under 60 seconds. Sorry to blabble on...

Now to the point... I have just realised that my bills were never over 50 pound in the past cause they never used to charge me a minute talk time for under 60 seconds calls, and it is only something they have introduced..... They claim to have sent me a letter sometime in August which I never got... They say to up my tariff, which I feel is unfair as I signed my contract, based on per second billing, which suited me fine as 750 minutes was ALWAYS amble..... Now!!!!! I'm receiving bills for WELL over 100 pounds... Do I have grounds to cancel my contract????