If they make a material change to the terms of the contract you have the choice to cancel. At least here in the UK, but that's a pretty basic contract law point in many jurisdictions- you only agree to what you agreed to! If you get an email saying "we are changing our T&Cs" read it carefully, they often give you 28 days or so to cancel in the small print, they don't expect you to read it! I used this once to negotiate a contract price increase once (not a mobile one though).