Information, tips and myths - No.31 - Wi-Fi calling
If you can't get any, or a reliable 4G or 5G phone signal wherever you are, then, depending on your phone and network, you may be able to use Wi-Fi calling. This will route your call through your router or a nearby Wi-Fi hotspot.
https://www.uswitch.com/mobiles/guides/what-is-wi-fi-calling/