Nokia 5800expressmusic
Not able to hear sum time

I bought this phone on 20th jan with my two friends... even they purchased the same mobile... but after one day i was not able to hear properly... the sound of the opposite person was on and off... the very next day i went to my dealer and he changed with a new device... but again i am having the same problem and evn my other two friend who bought this mobile are facing the same problem... we have always trusted this company so please dont break our trust and help us out with this trouble... waiting for your reply