Why doesn't he answer/return my calls?

Hey all! I need your help! I've been dating this guy for about 4 months now, and up until last week, we were working together. Now that we're not working together, it is so much harder to get in touch with him! We went out 3 days ago, and everything was great, but he hasn't returned my calls yesterday or today. Yesterday, he responded to every other text I sent him. I'm not trying to bug him, just calling to say what's up. I called him early this afternoon and sent him a text about 5 hours later, which he still hasn't responded to. Why doesn't he return my calls? And how can I get him to want to call me? Does this mean he's not interested? Should I give up?