You need to give 30 days notice to cancel, so unless you did that EXACTLY on your billing date there could be a further bill to pay and then after disconnection you will be refunded for any overpayment, as you realise, TT bill in advance.
If changing to another provider, it depends on when the service changes over.
Trying to run out your current credit exactly is not going to be easy to synchronise with all involved.
