In iOS 7 Apple added an additional permission for apps using the Microphone. The first time you launch Fongo you are presented with a dialog asking to allow Fongo permission to use the Microphone, please answer YES to this dialog.
If the caller on the other end cannot hear you, it may be that you may have inadvertently denied Fongo permission to use the mic, to check the setting, do the following:
Open the iOS Settings App and go to Privacy > Microphone > Fongo (Toggle to Green)
