Wi-Fi calling, also called Vo Wi Fi, is a great way to improve sound quality on your Android phone or iPhone during calls. When turned on, it runs your calls over your Wi-Fi connection instead of your carrier's cellular network. It works by routing your connection through an IPsec tunnel to your carrier's network; both devices don't need to have Wi-Fi calling enabled for the feature to work.
Pros of Wi-Fi calling include the ability to make calls in areas with little or no cellular coverage, such as dead zones caused by building materials like metal beams or concrete. It also offers better call quality by swapping cellular for more stable Wi-Fi, enabling HD voice. The feature is seamless: you turn it on and forget it, with no learning curve, no extra cost on most carriers, and automatic switching to cellular if Wi-Fi drops.
Drawbacks include dependence on Wi-Fi network quality. T-Mobile requires at least 2 Mbps upload and download speeds. Poor or congested connections can cause choppy audio or dropped calls. Seamless handover to cellular isn't always perfect, requiring Vo LTE enabled. Emergency calls over Wi-Fi are limited; carriers only route 911 over Wi-Fi when no cellular network is available, risking call loss if Wi-Fi cuts out.
Source: Engadget · Summarized by HeadlinesBriefing