Iphone 4S Mic is not working when in the speaker mode.

Its a new Factory unlocked Hong Kong version. i was calling someone yesterday and i turned on the speaker mode and i could listen to him but the other person could not listen to me. I tried calling again and again but all calls gave me the same problem..it works fine in normal mode. but mic does not work in speaker mode only...i tried with VIBER and viber has this problem too but sometimes viber works fine in speaker mode..but carrier calling never works in speaker mode.. i tried RESETTING it did not work.. should I replace my phone or restore it? also plz let me know if they will replace my phone with a NEW PHONE? because somebody told me they replace it with refurbished or repaired phones? miine is just 2 weeks used phone. i dont wana take risk