Did you check in the sounds area of settings to make sure vibrate is turned on? Also did you check the master volume in there (which then breaks down to different volume types. Then I would check and make sure the ringtone you were using wasn't deleted. If it was, you can run into this problem. Next try restarting the phone (turn it off and back on). Sometimes that is all it needs. If it is still doing this then check to make sure none of your apps has changed the sounds. It happens. Lastly, make sure that the calls are actually going through to the phone. Sometimes, if your carrier/service provider is having issues, the calls will go to voicemail and not ring on your end.