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Please check this first:
1. Press the volume up button on the side of the phone to make sure that the volume is not muted or turned down.
2. Check the ringer switch on the side of the phone to make sure the phone is not set to silent or vibrate mode. Toggling this switch should give you an onscreen indicator of the ring mode. Silent mode is indicated by a bell icon with a line through it.
Check the picture bellow:
Picture with instruction
3. Turn off Bluetooth on the phone if you are not using any Bluetooth devices at this time (Tap Settings > General > Bluetooth and set Bluetooth to Off). Please let me know if something of this works. If not, I'll provide you with the next step.