My iphone 4s isn't ringing but does display missed calls?

Whenever someone tries ringing it their phone makes a whistle and goes to voicemail. On my phone it will not ring at all, but afterwards show i have got a missed call from them, what do i do to fix this?

Added (1). Its not on silent, even if it was it would vibrate or show an incoming call for me to pick up but it doesn't just afterwards it shows a missed call

Is it on silent mode? Or is your sound off?

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