Try going into "Settings" then "Sound" and check the levels for each thing listed. You should also make sure you have a good loud ringtone. One that you can easily hear and will go on long enough.
When you set the sound levels and the ringtone it will let you hear what you are setting. As long as you can hear that, then your system is doing what it should.
If, however, the calls aren't ringing through and say going to voicemail instead (even after you checked the sounds) then you need to contact your carrier/service provider as the problem is likely on their end.