Reading the comments, it sounds like it could be:
1. You’re calling him too often
2. You call him “just to talk” and he a) doesn’t like talking on the phone or b) when you call him just to talk, it could just be chatter, and he doesn’t find what you have to say as interesting as what he’s doing
3. He’s busy at the time you call 1) working on something 2) playing video games 3) watching sports on TV
Quit calling him as much as you do, and let him have a chance to call you. When you do talk to him on the phone (or anyone for that matter), say what you need to say, and get off the phone. Unless you haven’t talked to someone for a few weeks, there’s no reason to be on the phone for more than a minute or two. Cultivate other interests, or hang out with your girlfriends so that you’re not sitting by the phone, waiting for him to call.