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Could you test if you could hear something from the earpiece of the phone while ringing another phone, e.g. call your friend's phone but tell him or her not to answer the phone - listen to the earpiece for any sound.
If there's no sound then:
1. Check that the volume setting is correct during the call - press the volume rocker buttons while you're in a call or while ringing another phone;
2. If indeed there is no sound even if the volume is correctly set, then it could be a hardware fault - take your phone to a service center and have them diagnose the phone for you.