I can hear phone calls only on speaker

Please follow the troubleshooting guide - If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

If following the steps above does not help, make an appointment with your nearest Apple Genius Bar or AASP, you should be able to get a repair or replacement for free if your device is under warrenty.

speaker must be damaged. speak to a repair shop or aple about this problem