I can't update the 3CXPhone clients through the dashboard on a new Debian install. I click download and it looks like its downloading and then still says I need to update. see capture.
Please try typing https://downloads.3cx.com/ in your browser and let me know if you are gettinga blank screnn and a "I am here" message on the tab on the top of the browser.
also make sure you are using a supported browser (google chrome preferably) clear your cache (ctl+f5) and try again