Try to think what you have to do soon to excuse yourself so the other end wouldn't feel too bad when you need to hang up. Then if you can, or want to, express how you felt during the overall conversation. Did you feel happy? enlightened? This way the other end would know if it is ok to call again next time and what to talk about as well. Then you say pre-ending "take care" or "see you again" or "see you next time" then ending words "bye-bye" or "goodbye" or "bye" I usually say bye bye because I feel goodbye is forever.