If they're a real friend and you're not an asshole you'll call them as often as they call you.
Whitelisting, and you can press a button to refund after the call.
No, there should be no option to refund. If there is it will become a moral obligation, many people will feel bad about not-refunding in whatever a case.
I mean when non-spammers but annoying people call.
It's about mass marketing spam, not people you know. People you know will call from a consistent phone number, and you can use time tested methods such as not answering instead of making them pay 0.10 to talk to you.
Most of my friends don't (and I love this!), they send me messages in Telegram and WhatsApp and we either discuss the matter this way or meet in person. If I felt like I am to call a friend I wouldn't mind to spend twice the rate and I would certainly love a half to go to the friend's account instead of just the phone company account.