VoIP phone systems need WiFi to work. However, you can still make and receive calls with VoIP if the internet is down. If you’re accessing a VoIP service from a smartphone, calls can be made through your cellular data network, including LTE, 4G, and 5G.  In some cases, you may need to upgrade your internet package or switch internet providers if your current upload and download speeds can’t handle VoIP calling capabilities.