You can start your thirty days notice in the last thirty days of your contract period and avoid any charges. But be aware that VM seem to have given all admin tasks to the most useless people on earth, so you'll need to check and make sure that they don't misunderstand that your cancellation is not immediate, but applies as soon as the fixed term comes to an end, and you are not liable for early termination charges. And even if they agree all that, check your final bill, wouldn't be the first time VM mess this simple task up.
If you're doing this in the hope of an outbound retention phone call with a too good to refuse offer, then be aware that isn't happening very much at the moment. If you don't get a call, then you need to have another ISP in place for when VM cut you off, or you'll end up phoning up to reinstate your service on whatever terms VM wish to impose - potentially "standard rate" pricing AND an 18 month contract.
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