Tech/comms question - voice call cellular vs app

[chassis]

Need some tech help, and I know bimmerpost is loaded with IT/tech/communication professionals, so here goes.

I was in Istanbul center city (lots of infrastructure) last week in a crowded place, mid-afternoon. I tried to place a cellular voice call and it failed multiple times. I chalked it up to network overload. I tried Apple FaceTime and WhatsApp, both were successful.

I was not connected to a local WiFi network. "Cellular data" was enabled on FT and WhatsApp. "Wi-Fi Calling" was enabled. I have full international access on my AT&T account and have successfully placed and received cellular voice calls to and from multiple countries for years, including in Turkey. I received a (cellular to my iPhone) voice call from a Turkey land line later the same day. iPhone 13 Pro with latest iOS and latest WhatsApp.

What gives?

What are the differences in communication protocols or data compression between traditional cellular and app-based voice messaging like FaceTime and WhatsApp?

Thanks.

[Markr]

We have seen similar results with automatic carrier selection on the phone when roaming, where the home carrier does not have a reciprocal roaming agreement with certain local carriers, and the automatic selection has unfortunately chosen one that cannot provide a service. For me, it has normally been fixed by manually selecting a specific carrier, or even moving 100m down the road.

We have UK contracts with just over 1,000 handsets/sims and have had the carriers investigate this type of thing a number of times, and get this response most times.