My settings indicate my phone should ring when I receive a call, but it doesn't. It does vibrate, though, but I'm not always with my phone to notice/feel it vibrating.
Making sure the ringer on your mobile device works is essential. Having the vibrate function work is great, but its not a good way to depend on being alerted for incoming calls. When did you notice an issue? Can you hear other notifications (texts, email)? Have you powered your phone off for 3 minutes and back on?