Is it me or the iPhone?

I bought my first iPhone last year around mid September. It was great until last march I noticed that whenever I make or receive a call the phone freezes. It turns all black and everything. Sometimes I have to shake it or press the home button or turn it off.

I sent it to mac and they replaced it with the new one. After two days, the new phone does the same thing again!

has anyone experienced this?

Well the screen is designed to shut off when you put it close to your face while on a call , not sure if thats what you are talking about.

Are you using a case that may be blocking the proximity sensor?

The proximity sensor is located to the left of the top speaker. Whenever you are on a call and it detects an object (i.e. your head/ear) it will shut off the screen in order to prevent accidental input...