how to fix facetime hesitation on iphone

My family all use FaceTime on iPhones connected via the T-Mobil carrier but with various internet/wifi services. Increasingly, FaceTime calls among us momentarily lose connection, freezing both screen and often sound, too, for a few to many seconds. One out-of-town member reports that not having this problem with other callers, only with family. Advice from the carrier and ISP focus on upgrading to the latest software versions, rebooting the iPhone, resetting the network, and ending other apps during a call. None of these recommendations works with any consistency. I suspect a buffering problem but don't know where to look or how to fix it.