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?