Who I have the same questioned this topic

I have been using 3-way calling since I've had this phone (2014).  Suddenly it hangs up when I try to add a third caller!
I've been using other apps that rely on this feature (karaoke and other music apps).  Their tech support says it's a known Verizonwireless issue.  VerizonWireless tech support says it's not a known issue.   I tried everything.  I resetted network settings a gazillion times, turned phone off/on, made sure I had the latest iOS, updated all music apps.  Same problem!