This particular feature is available for Apple devices only. It allows you to change the network your phone call is on.
Example scenario: you are using the Fastmetrics Mobile App for iPhone, but the call quality is not very good, because you are using a public or a non optimized WiFi connection. You can select to move it to your actual cell phone network (fees may apply). The below video shows the settings you must turn on, in order to have this feature available.
If you have voicemail set up via your smartphone network provider, the voicemail may pick up when you switch to iPhone. So, it is important to delay the voicemail on the smartphone native provider (let the smartphone ring at least 5 times).