Wifi-Assist Calling allows you to use your Wifi network to assist with making a call (please check that your carrier supports this feature). This setting can be really helpful for locations without great cellular reception. To enable Wifi-Assist Calling:
Settings > Preferences > Phone > Wifi Calling
Note: Wifi-Assist Calling requires some degree of cell service to initiate the call. This feature can not be used without a SIM card.
There are three settings for Wifi calling, 'off', 'cellular preferred' & 'wifi preferred'. The later two settings have this feature enabled and if there is a similar quality of reception between wifi/cellular, these preferences will dictate how the phone should try to connect the call.
Wifi-Assist Calls Notes
When on a wifi enabled call, there is a wifi icon above the call connected information.
If you leave the wifi area while the call is connected, the call will not drop as long as there is still enough cellular reception. The call will simply handoff to cellular at that time.