Skype ; Because my R.I.P. grandma was using it to talk to her relatives overseas just like she would use a phone, but it didn't cost an arm and a leg (unlike phone calls).
A genius product ripped my Microsoft. Have you used Microsoft Teams recently? Bad UI, hard to configure external hardware and good level of incompatibility, missing the good old "Echo / Sound Test Service". At a point I even installed Skype of my old Android but was sucking up too much battery.
and, after it was bought by Microsoft, the best spyware at the time. When it was running, the harddrive LED will be always on, just like network activity.
My friends now use Discord, and it's an acceptable substitute, like Skype it's always pushing a ton of features and updates I didn't ask for. ;-)