Not true (re WhatsApp will only text). The WhatsApp "app" downloadable on any smart phone, provides voice call, video calls, and transmission of texts, photos, and videos. Both phones must have the app. It's free, and easy to download, and easy to use. Both phones must have a connection to the internet for it to work. Either via WIFI signal wherever that may be obtained or using the cellular service provider's data plan. Nearly EVERYBODY in Africa uses WhatsApp, as it their service providers usually price "data", cheaper than airtime for calls. I spend a lot of time in various countries in Africa and do not purchase roaming from my home service provider, but I do purchase a local service provider's SIM card (costs a buck or two) and a cheap data bundle. You can usually do this right at the airport upon arrival in the country and will usually take no more than 5-10 minutes. Then I can communicate with anyone in the world for pennies using WhatsApp (again either through WIFI, or a low cost local service provider's data plan - no contract or recurring fees . . . $10 will buy enough data to usually last me three weeks ). From my own experience both in Africa and the USA, much of Africa - even fairly remote places actually have better cellular coverage than the USA. Heck there are many areas here in Wyoming and neighboring states that have zero network. I should note that WhatsApp is part of the same company that operates Facebook. So I also use "Signal" which is not commercially linked to Facebook, is end-to-end encrypted, and does everything WhatsApp does as described above. Signal would be my preference, but many Africa based contacts don't have the app so when communicating with them I use whichever app that they have.
I'm not trying to convince anyone to buy or not buy SAT phone service, but just to point out there are other options.