Calls drop repeatedly using International WiFi to US numbers

I’ve been using T-Mobile WiFi calling overseas from Korea, Germany, and Bahrain for over a year now. My usual practice is to connect my WiFi router to the high speed hotel network then use my iPhone X with WiFi enabled to make the call. Over the last year I’ve noticed a slow but steady degradation of the service. Calls placed via normal dialing to US numbers drop repeatedly. This doesn’t happen if I’m using local phone service (typically 25 cents a minute) or if I use WhatsApp on the same phone, which tells me the problem is probably on T-Mobile’s end. Is T-Mobile deliberately sabotaging international WiFi calling?