Lots of women wonder why their boyfriends don't call, some of it is just that a lot of guys are not in the habit of calling the girlfriend a lot, it's nothing personal. Some women might take this the wrong way and think something is going on behind their backs when there usually is nothing going on. But there is another situation where a boyfriend may not call..
If you are the one that always calls the boyfriend, he may be thinking why should I call her when I know she will call me. This is almost like having power over you. He can go about his business and wait for you. You need to turn the power over to you, don't call him. This will really mess him up not knowing what you are up to and he will have to call. And hopefully if you do this enough it will break him of this habit.
With you always doing the calling, you set into motion a never ending attitude on your boyfriends behalf of just waiting for your call, he probably means nothing mean in doing this but it is not making you happy. If the two of you went out on a date and he didn't call the next day, you just picked up the phone and called. He answered and everything was ok.
Break that cycle and make him call more frequently, after he learns to call you can go back to calling him again. Just make sure he does not revert back to his old habit.