Karma for listeners: Highly upvoted accounts can take precedence to talking to newcomers; they've proved that they can listen well, giving new callers a good experience. This will encouraging them to come back or even to sign up as a listener by following the good listener's example.
Karma for callers: If they ever call again, they will be able to talk with newer listeners. Perhaps you need to have enough karma to even be a listener. Why? Listening is a more important quality for the site to be successful. Starting out as a caller lets allows you to follow the top listeners by example, while keeping the trolls away from other callers.
Also, a low enough score can block the account for an X amount of time. Where X increases each time they get blocked. And I think more karma = more weight on your vote could help with regulation. Mostly because voting will take place much less often than on other social sites.
This is an awesome idea though, I hope it catches on.
Eg 1- someone going through a depressive phase hits up with a troll (ref: case where someone was in a chat room, streaming a video of himself just before he committed suicide, people in the chat room egged him on)
Eg 2- Someone is suffering a depressive/manic episode, but turns to this and is not directed to medical help immediately OR someone has an issue which would require them to go to a professional, but the listener is not trained/aware and hence can't give them critical advice
Eg 3- Young adult who has had a traumatic day turns to this service, gets put in touch with someone who has strong beliefs. (or even mild ones)
Eg 4- High Karma person is taping calls of people talking to them
Okay. Waiting for someone to call... ... ...
This is the most inefficient way to rickroll anyone ever
I still hope you have some throttling in place though because 1000 concurrent users will cost you $150/hr...