Calls

There is no need for you to "contact iCloud support." Apple will never cold call a customer with that, or any other information. You can safely ignore the call/block the number.

If you did respond, and provided personal information, be sure to consult: If you think your Apple ID has been compromised - Apple Support

Edit: That Support Article also contains a "Contact Support" link.

You are welcome...just keep blocking them. As long as you don't respond, eventually they will give up. You should also consider using "Turn on Silence Unknown Callers."