Yeah, it's wierd, I have the same problem. They all call me back immediatly after seeing my missed call, but will never call me just to call me.
The one time this one girl did call me she was like "Hey, I'm so sorry to bother you, I know you're probably busy or at work or something (It was like 8 pm, don't know why she thought that), but I just had a question about that quiz tomorrow..." or sometimes she'll say "o are you busy now, I can call back later..."
And she even told me in person on one of our dates that she felt bad calling me because she thinks she's bothering/irritating me. I'm like WTF?! I want a girl to call me for once!