What reliable alternatives would you suggest?
I am not affiliated, I just like it.
I do like mumble and ran my own server for years, but client wise it is not as easy.
These should be added soon™, though.
What we need for the internal communication (within the company) is a simple IM, integrated with ticketing system (a bunch of generic rooms, per-ticket rooms, linking from tickets, ...). We're pretty much set on using Mattermost for that, which has the advantage of being self-hosted.
For voice (typically group calls with other engineers) we'll probably stick with Skype for now (I the Android app on my phone for that). For meetings with customers we usually use a regular service with dial-in anyway.
We don't do video, and I don't think I've needed screen sharing in the last few years.
For video though your options are definitely more limited, keep an eye on Riot, I'm not sure that it's ready yet, but when it is it will present an extremely good option. If you only need mobile, Signal does it today and is probably one of the most stable and secure products around, can't vouch for the video quality though personally.
Google Hangouts.
But I guess we don't really need the video calls that much (After all, who'd want to see video of engineers working mostly from home, right?) and the ability to self-host and integrate Mattermost with the issue tracker etc. is quite neat.
Web-based, supports notification, Slack integration, custom urls.