My iphone5 does not always ring when there are phone calls.

My iphone does not always rings. I just realized since I had a phone interview today at 3:00pm. I waited 30minutes before with the phone in front of my face, and 2:51pm, there was a call from another person, I picked. Then I waited and waited, until 3:12pm, I thought something went wrong. There were missed calls and a voice messege displayed on my screen at 3:00pm. The phone never ring!!!!!!!!!!!

I Just missed a phone interview! OMG!

Could you give me a solution to solve this problem?????