Ah, Alan Wagner, yes.

iPhone is usually automatic when connected to wifi, however I believe you need to utilize the call option via the FaceTime app, not the phone app to initiate a VOIP call when out of service but connected to wifi and basically bypass the need of communicating with cell towers to relay the call as at that point it uses your contact and your apple IDs to make the call over VOIP. For android, there are third party apps you can install that allow you to make VOIP call, anything such as Skype to even Microsoft Teams would work. Of course these require the appropriate accounts and your contact to have an account and utilize these same apps as well.

There are devices that could be installed on the plane that emit the appropriate cell tower frequencies and allow phones to connect to them and then broadcast their data back over standard IP as well, though I haven’t encountered this on a plane yet but I know such options are already in use by some consumers and businesses and could see this be implemented on a plane to make both calls and data hassle free for customers in the future.

A lot of phones can make calls over VOIP, iPhones in particular. This is done via in-cabin wifi, which is available in air thanks to satellite receivers/transmitters installed on the plane. People have making calls for years now thanks to this service while in-flight, even though some airlines prohibit this.