If you have not already, go into your settings app on the main screen and select Sounds
In the Sounds menu, set the ringer and alerts to maximum (you mentioned you cannot hear it ring)
Make sure the tab is set to ring and not vibrate on the left top side of the phone.
Make sure the volume is set to maximum by toggling the volume tabs on the side of the phone.
If all this fails to work, try a software update of the firmware, (you would be surprised how many problems this can fix)
Having done these steps, it is possible that the internal speaker is itself broken. Assuming it is a hardware problem there is nothing more to do other than to take the phone in for repairs.
Here is the link to apple's Iphone repair website;
https://www.apple.com/support/iphone/repair/
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