Two things to talk about with girls are: Things you are interested in, and things that she is interested in. It really helps if you have an active life with things that you enjoy. If you are happy with your life it makes you an interesting person to talk to.
Think about people you've met that always seem to have friends. One thing they have in common is that they are truly happy inside. This is a HUGE magnet. Everyone wants to be around a happy person.
Be yourself, don't try to impress them. People who try to boast or brag or make themselves sound important are always a turn off. Just be sincere.
Have a sense of humor, try to see the funny side of situations. If you trip or do something embarrasing it's ok to laugh at yourself. Girls feel kindly towards a guy who isn't trying to act macho and isn't always trying to impress them. You can even smile and make a little joke about how nervous or shy you are.
Don't try so hard. Just treat them with respect like you would treat a favorite sister. Enjoy their company. Laugh a lot.
About phone conversations - dont' try to carry on long phone conversations with girls you don't know well. That's hard for any guy! You can't see their body language to gauge feedback, and you don't know if they're in the middle of something. Use the phone to ask them out or whatever you called for, and keep it short. Be sincere, be polite, and be short! That's not the time to have long involved conversations about politics. Save it for face to face times.
Hope some of this helps you, you sound like a nice guy!