Hi,
Have you checked the volume level when you are making a call?
Have you tried using the loudspeaker option when making a call?
If these are both OK then perhaps the receiver in the phone is either faulty or has become disconnected from the systemboard.
If you do not wish to use the loudspeaker option try using the earphones with the microphone attachment and if it works OK as well use this until the phone is repaired.
If you still cannot hear after trying all 3 options listed above then there is a problem with the voice reception circuitry in the phone.