Try 2 talk 2 him and figure out y he isn't calling you back. If he doesnt give you an answer or just says something like "its none of your business" then he might be doing something that you probably wouldnt like behind your back. But just try 2 get through 2 him and if you can't at least you tried.
Give him a taste of his medicine and don't answer his calls too.He will know how you feel and possibly change his ways,if he doesn't HE IS A JERK!
Look of your boy ant answering your calls check his scheeld and if you know it his CHEATING