I had paid up front for line rental saver a few months before BT last hiked their prices.
When they upped the broadband price, I terminated the broadband only, and kept the landline - and had an assurance (in writing, from the chat guy in India) that I could later terminate my landline with no penalty.
About a month later, BT then tried to tell me that by removing broadband from my package I had entered into a new minimum term contract for calls and line rental (at £0/month, but with the implication that I would have to start paying line rental to BT again once my line rental saver expired).
I argued the toss, and eventually got their CEO's complaints team to agree that I could indeed terminate with no penalty.
I terminated my line rental with BT when the line rental saver was up and moved to the same provider as my broadband. BT then tried to charge me for early cancellation of the contract. I kicked off again, and got the full amount refunded eventually.
They are twats. But if you stick to your guns, threaten to take them to court, and don't ever back down, you can walk away from your contract without losing anything (apart from a few hours of your time - but for the principle, it's worth it...)