IPhone 4: during calls the other person can't hear me?

I believe it started when once I dropped it and externally it was alright. However, in my calls the other person can't hear me. It's definitely NOT A MICROPHONE PROBLEM, as the mic works perfectly in every other app (including Skype, Viber etc).

I tried the youtube tutorial that suggests to put cardboard inside by removing the back cover, however it works only sometmes, and sometimes it helps to press the back. (not really effective)

It is really frustrating and I'm ready to switch phone, cause I simply can't use my iphone for regular calls. Any ideas?

Yes iPhone 4 has the problem if you make a call form internet
but if regular call is so the u better show it 2 the repairer