International calls not connecting

I have a question . . . my wife is from Costa Rica, and she has family down there. It has happened several times now, that when someone calls from down there, Flor’s phone rings and a picture of the caller appears on her screen . . . but when she answers, no one is there! So she calls them back using a phone calling card and they confirm that they can hear the phone ringing, but then it just goes “blank”! The phone rings but they are not “connected”!

They are both using “What’s App”, and there used to be no problem at all when Flor had the “previous” phone (an Alcatel one touch). She also had the “What’s App” app on the Alcatel. And they all have it in Costa Rica.

Maybe you can think of a reason for this to be happening, but the only thing I can think of is some setting in the “What’s App” program. Do you think this may be a possibility? Can you think of anything else? This is a Motorola G4 cell phone.

Thanks for your attention,

Brad