In many circumstances, WhatsApp works better -- like, a lot better -- than normal calls. Also it's often subsidized.

They probably could have paid extra money out of pocket to call normally and get a substandard experience.

Normal telecom services do not offer free international calls or video conferencing like WhatsApp. You also would need a sim card and phone plan. WhatsApp just requires wifi.

International calls might be too expensive unfortunately.

The phone call might be right before ventilation rather than last words too.