Do you know these girls very well? You might do better to communicate with them in other ways - in person, instant messenger, e-mail. You might also want to make a list of topics to discuss with them if you find the conversation slowing down. (Sure, it sounds kind of dorky, but it's worked for a lot of my guy friends.)
When you do speak on the phone, try calling for a reason - like, "Do you know how to solve this problem from the math homework?" or even better, "Do you want to go see a movie this weekend?" Like Sukotto said, when you say good-bye, be sure to make it sound like you have something else to do.
Don't worry. It takes practice, but these things WILL come to you more naturally someday!
As for whether girls "understand"...well personally, I have always been attracted to the more introverted types. Shy guys tend to be much more sensitive, and I find awkwardness adorable.