There are many factors that can influence the FreePhoneLine calling performance: network connection, sound card drivers, headphones quality and so on. In order to fix the issue I suggest you both to close all unnecessary applications that use the Internet bandwidth and stop any file downloads. Also, update your sound card drivers and use a quality headset with a built-in microphone.
If the problem persists, create an account on the official web page and download and install the latest version of the software. Alternatively, you can use another VOIP program from Software Informer database.