For my family member who moved out of country, signal’s voice quality was not very good. WhatsApp’s quality was better than a regular phone call, very impressive.

Also, outside the US WhatsApp is some kind of standard the way rich Americans think iMessage is. Companies will ask her to contact them on WhatsApp, so she’ll have to have it regardless of your personal involvement.

There’s also jitsi.

I discounted this theory the first time I heard it because it’s too cool.