I clearly installed ungoogled-chromium version 81.0.4044.92-0.b484ad4 from GNU GUIX repository, checked that voice/video calls works in several web meetings clients like Zoom and MS Teams, then open https://web.skype.com/ and got the message that "Audio and Video calls not supported on your Browser"

To Reproduce
Steps to reproduce the behavior:

1. Open https://web.skype.com/
2. Login with skype credentials
3. Try to call to the "Echo / Sound Test Service"
4. Call button is not active and write "Audio and Video calls not supported on your Browser" when your mouse over this button.

Expected behavior
Calls should work fine.

Environment (please complete the following information):

• OS/Platform and version: GNU Guix f6145358c7d33e73c0708bbe400e6e8e65512ef3 (April 13 update)
• ungoogled-chromium version: 81.0.4044.92-0.b484ad4

Additional context
IceCat with same User Agent string as in ungoogled-chromium is able to make calls.