My IPhone 7 Plus is not letting me recive calls or call people

Whenever someone calls the calling screen comes up for a second but before I can hit the answer button it goes away. Then when I try and call them back it's goes to the calling screen but before it can even ring once it hangs up. This has happened with many different people so I know they arnt just hanging up or deneying my call. My carrier is AT&T.

How can I get a vendor of phones to sell?

Like you see how all those peoples on ebay or these sell online type of websites that be selling all of those phones like iPhones, samsungs, lgs, zte, etc anything that's a phone. How do they all be getting all of those phones to sell and make a living? Like what's the trick? Because I know they can't just be spending thousands and thousands of dollars to get all those supplies of phones to make a living because it's like what's the point your spending the money on the product on that you just made the money with? So what's the trick? Because I want to run my own business online and sell phones and make a living? But how do i get started? Please someone help! Any advice thank you.

Music volume with headphones on IPhone 5se keeps going up and down on its own?

I have an IPhone 5se and when I'm using headphones the volume cuts out going up, plays, and then cuts out again and goes back down to where it originally was. It does this very randomly probably every minute or so. I don't think it's the headphones as I plugged them into my laptop to listen to something and it was fine. Has anyone else experienced this problem or know what it is?