As I understand it, the National Do Not Call Registry clearly does not prevent a company that you already have a connection with from calling you. From the linked FAQ:
Companies can call you if you’ve recently done business with them, or if you’ve given them written permission to call
I don't know all the details, but I suspect that completing a web form is treated as "given them written permission to call". That means the initial calls are legitimate, which makes sense since you asked the company for information, even if you didn't expect it to result in sales calls.
However, the FAQ also says:
But if you ask them not to call you, they have to stop. Be sure to write down the date you asked them to stop.
If they are a legitimate company, then if you tell them clearly "do not call this number any more" they will stop. Unfortunately, many companies are not legitimate, law-abiding companies and will keep calling anyway.
My recommendation:
- If you have not already done so, sign up for the Do Not Call Registry. It won't stop everything, and in particular won't stop overseas VOIP-based scammers, but it will cut down on some calls, and it provides a legal basis for stopping calls.
- Each and every time they call, inform them that you want them to stop calling your number, that you are on the Do Not Call Registry, and that you will report them to the Federal Trade Commission.