Why would my phone ring sometimes and not other times?

[iMore Question]

My phone is on ringer. The volume is all the way up. Intermittently, the incoming call will not ring. I would say it happens about 20-25% of the time. At the other end the person goes directly to voice message. I am not on the other line. I will look down to see that somebody called but the phone did not ring. What could be causing this?

 

[bamf-hacker]

Generally that is due to poor cell reception. Do you have good signal?