You can get monthly snapshots free of charge which receive some testing compared to rolling builds. They currently have 1.3 RC builds https://vyos.net/get/snapshots/
When Slack launched video calls, it was using Chrome's own implementation of WebRTC which is called Plan B. So it worked in both their app which is electron based and Chrome. Not sure if it worked with chromium derivatives like Brave, Vivaldi, etc.
Firefox rolled out the standard based WebRTC implementation and chrome eventually did as well, but companies like Slack and various others didn't migrate instead just threw an error message saying calling isn't supported in Firefox and instead use their app or chrome.
But their browser check still remains today and Mozilla started to spoof the user agent as a workaround so that Firefox users can make calls as Slack aren't willing to remove the check or properly add support.
They tried but Slack weren't interested. Update from 2 years ago
To summarize my current understanding: Slack's video conferencing service only support Chrome's non-standard Plan B format for WebRTC calls. And our attempts to talk to Slack about changing that were kindly rejected (we met with them shortly after they had launched the calling feature). And my repeated questions in public events about adding Firefox support were also turned down.
I have seen people mention that they have tried few more times to contact them, but Slack weren't interested.
My understanding as to why it works now, correct me if I am wrong, is because of Slack moving to using AWS Chime SDK for calls which has Firefox support and just faking the UA to Chrome tricks Slack into loads the necessary SDK/JS for calls and Chime SDK takes care of the rest.
It would be nice if it would integrate with a 3G/4G dongle. Huawei dongles (maybe ZTE as well?) already have an HTTP endpoint to get SMS and also their USB dongles have no battery, so it is much safer to leave it plugged into a wall 24/7 than a phone.
Most phones these days do have battery charging cut off, so they should be safe but still prefer to have one without battery for leaving it plugged in 24/7.
They now have a dual stack EC2 API endpoint. But you have to go out of your way to use as it is on a totally different domain, and also it is limited to few regions. us-east-2 region for example