I'm wondering, how do you ensure the people who have called you have indeed paid for phone support? Do you just assume callers have since the support number is

I'm wondering, how do you ensure the people who have called you have indeed paid for phone support? Do you just assume callers have since the support number is not published anywhere? Or do you ask for a username/account number or something like that? I've never used phone support for something that isn't targeted to all-consumers-but-none-in-particular.