Info on Contracts from the Consumers' Association:
link
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What is a contract?
When you engage a service supplier you create a contract between you, as a consumer, and the service supplier. A contract is an agreement between two or more people that is enforceable by law. Contracts may be written or oral and are made up of terms; some of which can be implied terms. Contracts may differ in many ways and there are no hard and fast rules governing what terms should be in a consumer contract. Terms in consumer contracts, however, must always be fair and clear to the consumer. Anyone involved in the contract should be clear about what their obligations are under the terms of the contract."
Basically, this means that you need to check the Terms and Conditions of the customer contract that Clearwire use, that you agreed to when you signed up to their service.
This should determine whether you owe the money, due to the contract still running on after you stopped paying. The T's & C's may state that written notice of termination is required. Although a verbal agreement should be enough to terminate any contract, many service providers' contracts do require termination in writing, to avoid misunderstandings or malicious terminations by third parties.
In fairness, Clearwire did withdraw the service at your verbal request, so you might talk to the Consumer's Assocation to see if this alters things (in doing this, they may possibly be deemed to have terminated the contract themselves, but perhaps not).
Even still, given that you did not/could not use the service after verbally terminating it, they should be prepared to leave it be but, knowing their reputation, I doubt that they will, as their policy seems to be to get as much money from their customers as possible, with the least customer service provided...