In Eastern Europe, Viber is pretty popular. Also, many telecoms in Bulgaria, for example, don't account Viber data usage towards your quota, so, even people on the lowest tiers get unlimited free video and audio calls. But some offer this for WhatsApp traffic and many mix and match it with Facebook. So, Microsoft really doesn't pay much attention to Skype - it was the most popular service in Bulgaria, but now almost nobody uses it due to these promos.
I assume those data waivers are set up through deals between the telcos and the companies behind those applications. Microsoft certainly has the resources to arrange similar deals, but they seem to have given up on Skype years ago. Absolutely baffling, especially with the recent valuation of Zoom.
