Apple iPhone not answering calls

I have found a slight bug/glitch/disturbance in the force with my iPhone. I work in the field a lot and need to ocnserve battery power so I shut off the screen whenever possible. If I shut the screen off using the top right hand button WHILE checking Emails I will not receive phone calls. My friends/family/customers have complained that they get a message saying "caller is unavailable" and then it hangs up. If I make sure to exit the Email function and then shut the screen off I get all my calls.