Loss of audio in earpiece

I own a Xenon GR500.

Since this week suddenly the audio in the earpiece looses the audio while talking. The other person can hear what I am talking but I cannot hear.

After swithcing off and on the phone is back to normal. The earpiece works. But looses the audio again suddenly.

During this loss of earpiece audio period, sound can be heard through speaker phone.

Please let me know as to how can I identify whether this is a hardware or software problem? and how can I resolve the problem?

I did not mean the Handsfree earpiece, the phone's earpiece which is placed on the ear to listen.

I did not try the Handsfree yet.

The speaker (phone) works without any problem.