Sometimes I can't get incoming calls with my iphone 5c?

I live in a dorm and I guess it's logical that the walls/steel are blocking the signal or I'm interfering with other signals of different carriers. I have sprint. I have full bars and yet most of the time, I don't receive any of my incoming calls. My friends have said that it goes to voicemail after a few rings.

I went to the apple store already, and the guy tested my phone. It worked perfectly fine (I'm a bit biased about the place. As if everything will work in there). So I took my phone back and it still has problems. Sometimes it takes a while for the phone to make the dial sound when I'm calling others.

I have only dropped my iphone 5c probably three times, but I have a strong protective case that covers all over it. So far, no damages (knock on wood).

What's the problem here? Should I just keep taking it back to the apple store? It has a one year warrenty, but I can't seem to prove that somethings wrong it right then and there.

Added (1). When I mean by dropped, it's not a DRASTIC drop. It might have fell no more than 2 feet. My phone case has protected it.

Dropped 3 times? Who knows what damage you have done. I would just make sure you close all background apps, that may be consuming your resources.

Go to Apple again, complain that it doesn't work. Tell them you were here before but you still see the problem. They will refund it or give new one to you.

If the phone works in the Apple store, it will be considered by them that the phone is fine.

Apple provides warranty only on their appliance and not on the area that you live. The problem that you are facing, as even acknowledged by you is due to the building. So it is unlikely that Apple might help you. Simply saying, if you go to Mars and say that your phone has no network and is not working is not a valid reason to prove that it is faulty.

Do you have apps like Caller ID, Truecaller, Call Filter, etc on your device? These apps purposely delay the audible tone during which they search for the caller or block list etc.

Take it back to them once more. This time try a different Outlet/Center. Try to reproduce the problem as many times as possible beforehand so that you may understand when does it occur. It will enable you to explain them the problem better.

You can try a simple check, have you enabled the auto call forwarding feature? Or auto voicemail?