Handover allows you to switch from VoIP over Wi-Fi to a cellular voice network call while on a call (Note that the other party will hear silence briefly.) While on a cellular call, tap "handin" to switch from the cellular voice network to VoIP over Wi-Fi.

The Automatic Handover allows VoIP calls over Wi-Fi to be automatically switched to your carrier's cellular voice network when the Wi-Fi connection deteriorates.