When you download the SkyBell app, your phone will ask you if you want to allow SkyBell to have access to your microphone. You must tap "Allow."
If you begin using your SkyBell and everything seems to be working fine, but when you push "Hold to Talk" to begin talking to the person at your door and they cannot hear you, this probably means that the app does not have access to your microphone.
In order to change this setting on iPhone, go to your home screen and open "Settings" ~
Settings -> Privacy -> Microphone -> Put SkyBell on green