Why cant others hear my voice while calling?

I took iphone4 from my friend who used it in UK .i got it factory unlocked by means of IMEI no. After unlocking i updated my IOS from 4.1 to 4.3.1. After that problems started,people cant hear my voice on calls but i can hear them, more over i can hear my own voice in earpiece when i speak. I tested on VOICE MEMO whether my microphone is working or not, it is recording my voice and i can play it. I am very much worried of it regarding this problem.When i tap with my finger near headphone and microphone jack my voice is going on calls but for a few seconds only. Please somebody help me. I just want to know is it software problem or hardware problem.

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