> What are the chances that the person you spoke to had any knowledge of the action you are talking about?
Little to none, but this is by design. If they had made such a decision, tracing it to them is difficult. Easier to tell the phone reps nothing and push the blame onto either end of the transfer rather than on the path in-between. How many people will pursue this past just venting about it on the Internet?
> For that matter, how do you KNOW that the ISP did indeed terminate your large encrypted file transfer?
If it's a single transfer, then you really can't. That said, you can establish a pattern. For example, if your connections always terminate around the 20-minute mark and this happens regardless of the other end of the connection (or the network path to that end-point), you could make a compelling case that it's the ISP's fault at the very least.
EDIT: For that matter, how do you KNOW that the ISP did indeed terminate your large encrypted file transfer?