NonprayingMantis: IANAL, but for their to be a contract there has to be some consideration.
http://en.wikipedia.org/wiki/Consideration
"Consideration is a concept of legal value in contract law. It is a promised action, or omission of action, that the promisee did not already have a pre-existing duty to abide by. It can take the form of money, physical objects, services, or a forbearance of action. Both parties to a contract must pass consideration to the other party for there to be a valid contract."
If she has had nothing from the contract (no discount, no new hardware etc) then you could find that the contract is invalid.
NB.? Consideration does not usually have to be a fair value, it can be anything you like ($1 for example), but it must be there on both sides.
You know what I'm sick of? Some dickhead on a forum who has done some basic level 1 law paper or similar and thinks they're an expert on contractual law. "Consideration" isn't some code word for a free phone. She gives them money, they give her a phone service. There's a term involved. Have you ever leased a property? Notice how you are committing yourself to a certain period? This occurs for the benefit of both the seller and the purchaser - there is security and comfort in knowing you have a certain deal for a certain period of time. That doesn't mean your landlord has to throw in a free boat or something.
The bottom line is she signed a contract and she committed herself. Presumably if this were the other way around (eg Vodafone trying to weasel out of the contract somehow) then you would be a lot less cavalier in your attitude toward the agreement.