This is mentioned in the faq about porting numbers, but we also address it when someone requests a port so they don't have a bad experience (this is why it's currently a support request rather than a UI capability, until we've got that workflow working for the customer's overall experience). We haven't had any mobile lines cancelled yet, but we've only worked with tmobile, sprint, and AT&T so far. It's usually best if you work with your carrier at the same time and we can provide some advice on that for those that request a port.

This might be more prominently featured as we go forward.