I used to work at an outbound call center in an electricity company. We would call customers whenever the billing systems were updated, or if your personal information hadn't been updated in so long. We had to call people who were on a list to be notified when their meters were going to be checked. We had quite a few reasons to call people, and we would always have to go through personal information to make sure we were talking to the account holder, along with account numbers and passwords, etc. We would also give out a different number to call back on since the phones we called from were outbound only and they didn't accept inbound calls.

If we ever had rude customers that told us to "fuck off" or got very aggressive, we would put in a note entry attached to your account. So anytime you call back to make payments or anything, the note saying "customer was very rude and was using profanity" would pop up for the representative. Your account would be pushed to the bottom of the list for notifying, so sometimes you wouldn't receive a call at all if we didn't have time. The address associated on your account would be "blacklisted" which meant you would be one of the last houses they check the meter at (which usually results in estimated bills meaning you'll more than likely owe more upfront). When you call in to pay bills and they see the note, they will put you on hold and take other calls until a supervisor could speak with you. That would usually take anywhere from 30 minutes to an hour and a half on hold.

So your best option is to be polite to "unsolicited" calls, whether they are legit or not.