Internet too unreliable for voice calls even

My (up to 600Mbps) internet connection is not capable of maintaining a voice call – either on a video chat with the video turned off, or on my cell phone with wi-fi calling.  In both cases it may start off ok, but then become intermittent, broken up, or drop all together.  This is seriously impeding my work.  Speed tests vary wildly – a moment ago it was 70, now it is 15.  My Wi-Fi signal in the house is strong (I’m an engineer, I tested it), and in any case I have tried using a wired Ethernet connection in place of Wi-FI and that doesn’t make any difference.  The modem is compatible.  Everything in the house seems fine.  How can I diagnose the issue?