When a user moves into Teams Only mode, the Skype for Business client becomes a “Meeting Join Only” client, meaning it can join meetings but is no longer a calling or instant messaging client.
This is how it launches:
And if you click “Start Skype for Business” you get a client that only shows history and upcoming meetings:
If you join a meeting from the meeting tab or, more likely, from a link in a meeting invite, you will always be promoted with the “Join Meeting Audio” choice.
You are not going mad, even if you have your default set in SfB client settings and “Before I join meetings, ask me which audio device I want to use” unticked. It will always prompt.
Unfortunately when SfB client is in “meetings only mode” it is hardcoded to always prompt. I believe this is because initially the meetings only mode removed access to the SfB client settings and I assume someone wanted to be prompted by default.
I believe the registry key that controls this behaviour is AllowOverridingDeviceAtJoinTime (thanks Yoav), but setting it, like ticking or unticking the tick box, has no effect.
I think I can live with it, but if you find it annoying I’ve setup a user voice here: