Not sure about this now, but I think that a case like this may have left you with a slight problem, due to the fact you agreed on the phone to extend your contract, and as such you have a verbal agreement. Contact Consumer Direct for the latest consumer laws on this. click here
The other alternative is to see what is delivered, and if BT enclose any paperwork for you to read and/or sign.
You might want to read this click here but I think that you may find it doe's not apply.