Why does my iPhone not make a sound when I receive a text from an unknown number?

When I get a text message from a number that is not in my phone, my phone lights up and shows that I have a text but does not make a sound or ring. Can I change that somehow? I have an iPhone 5.

Setting>Sound
It could be on silent their should be a switch on the side push it up and you will see a bell
If its not on silent just turn the ringer up

I have an iphone 4s the phone is set to loud and the mute switch is off, when my phone receives a call it will ring but when i receive a text it doesn't, also the keyboard is silent and the music is silent on playback, the keyboard clicks are set to on and everything else is set so i can hear it! I never had this problem with my iphone 4, can any body help?

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