I agree with what you're saying. They got the phone from Carrier A with the expectation the phone plan went with it. Once the phone is paid off, they can take the phone to Carrier B. Since they phone is basically bought on an interest free loan, the interest is recouped by the plan, and the collateral for not paying is a loss of the phone plan and use of the phone. To leave the plan, payoff the phone.
That does require that, the moment the phone is paid off, it should be automatically unlocked. There shouldn't have to be a request or additional waiting. And the customer should be notified that it's unlocked along with an explanation that they can now use the phone with any other provider.
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