I guess it depends. There are some people in my life who if I leave 3 messages for with no return call I know without a doubt it's not intentional and they still love me and everything is good.
However, there are more casual friendships where if I left 2 messages without getting a return call, those people would be moved to the back of the line, in my mind. My way of thinking at this point would be -- if they wanted to be in communication with me they will call at some point. They now know I've make the effort with them and the ball is in their court. If they never call, then they are telling you exactly how they feel about you.
I don't really buy that people are so busy or distracted that they don't contact people they want in their lives. However, people ARE so busy and distracted that they don't contact people they either don't care are around or they don't want around.
From my perspective, I don't want to be friends with someone who can't be bothered to return my call or email or whatever. No thanks. The reason I'm like this is beause I spent years trying to forge more meaningful friendships with people where very fair-weathered friends. It was exhausting and stressful and I finally realized I'd rather have a couple GREAT friends than lots of friends who couldn't be bothered.
I'm sure you have not offended anyone - and I feel this way too - all the time if I say something wrong or it comes out wrong.
How do you know? If they call you back they want you in their life. If they don't call you back, they don't want you in their life - at least not in any meaningful way. Would you ignore someone again and again that you wanted around?