I have only seen that the client is using http and https (more infos here). But to be on the save side you could run fiddler and check which ports will be used here (including hostnames if you need).
However we had customers who reported the same, so here is the summary for the issues and solutions we collected so far (as we run a very big environment, there are a lot to share):
- locked down rights on the PC & Internet Explorer which caused that the correct plugins couldn´t run correctly
- incorrectly configured headset, where the MIC wasn´t allowed to be used as recording device in the system settings
- Locked down USB ports which do not accept the headset they wish to use. Strangly audio was working, however not the MIC, after whitelisting the USB device by the local IT in there USB management environment it started to work.
- One customer hat a damaged browser installation. Google chrome caused exactly these issues reported, however IE was running fine.
- For some customers not all Microsoft cloud IPs for Office 365 are open. They initially opened some of them, however the cloud was/is changing. This caused that the could join, however audio wasn´t working due to that issue.
- A software on the PC blocked audio & video streams. We do not got more info's here as the IT from that company re-configured that software but according our info's it was some kind of DLP (Data Leak Prevention).
By the way, the best would be if you run a test on your side as well to ensure that there is no configuration issue on your side somehow. While you do that you could run fiddler and validate the needed ports and hostnames/ips.