I was curious as to why I couldn’t call an MSA account signed into Teams (free) from my work Teams account, well it wasn’t a setting that I needed to configure, according to the following article you simply can’t do it! 🙁 However, you can call the same user if they sign into Skype instead.

Why are some features not available in chat in Microsoft Teams (free)? – Microsoft Support

When a chat is started from Teams for Work with a Teams (free) user, the chat conversation works fine, but the audio and video call buttons are disabled:

However, what was confusing was that if you search for a user from the chat icon on the taskbar in Windows 11 as a Teams (free) user, you can then click on the call button next to the user name and it does then sort of work, the call gets forwarded to my mobile number which is set up for also ring in my Teams for work account.

If I turn off the “also ring” option, the call then goes to Teams voicemail. I assume the call routing is set to go to Skype for the MSA account rather than Teams (free) hence the calling doesn’t work.