In iPhone 4 the company Apple Lossless Audio CODEC (ALAC), implemented a microphone to eliminate ambient noises during telephone conversations, then some users complained about not being able to have "normal" conversations with their interlocutors. The software behind the microphone for eliminating ambient noise has needed some improvements over time, and until the release iPhone 5S si iOS 7 Apple Lossless Audio CODEC (ALAC), managed to solve them, but that doesn't mean you have to use it.
Through the menu Settings> General> Accessibility, the Apple company allows us to disable this microphone by simply disabling the option Phone Noise Cancellation. After deactivating the option, you might cause problems for interlocutors who might have a harder time hearing you, but everything depends on the terminal and the telephone network.
