Yes, I am in a "first-world" country where telephone calls are cheap and the menu of included features are many. I did not intend to sound "snarky" - often the e-mail type communication can sound more aggressive than what is intended.

In the type of country which you are describing, even getting a basic "land line" can be difficult. Sometimes you have to bribe the telephone guy. He disconnects a line from some other guy and gives it to you. Then that other guy with "dead air" instead of "dial tone" has to come up with a bribe to get the telephone guy to steal a line from somebody else! And so it goes...

Surprisingly, the cell phone networks in some of these under-developed countries are quite good.

Instead of some app that can run on some ancient computer, I would focus upon using an app that the guy in this 3rd world country can run on his cell phone. The available "computer power" in the cell phone is likely to exceed the power of an ancient computer.

It is unclear to me why one of the many commercial apps won't work?

Even if you have some basic encryption (and many of these apps do), the government of this country will know who that person is talking to!

As a reference, my internet connection can support about 3 simultaneous video and audio connections. Whatever site you decide to host this applicaion will need more bandwidth than that. This typically means that your home computer cannot do it.