iPhone 4 Microphone not not working on voice calls but voice memos, viber, skype & facetime work just fine!! Any suggestions?

Hi Everyone,

I have encountered the following problem which I think a lot of other fellow iPhone users are also encountering however mine seems to be fairly pecuilar.

Here's the issue;

When making or receiving voice calls via my network operator's service, no one can hear me, i.e my audio input. I can hear the person on the other end of the phone fine but they can't hear me whatsoever.

However when I use the likes of viber, skype and facetime, my audio input is perferctly fine and the person on the other end of can hear me.

Note both viber and skype calls work just fine and 2 way communication is audbile regardless if I have a 3G or Wifi connection. Plus I have full singal bar strength when near a mobile tower so that elimantes antenna issues.

Also have taken alternative turns to cover both the top mic and the bottom mic during a viber or skype call and my audio input can still be heard.

I've done a reset, a hard reset of the phone and a reset of the network settings, still to no avail.

Have updated iOS to the latest version 5.1.1

Any suggestions would be much appreciated?

Thanks