As far as I can see from here https://answers.microsoft.com/en-us/windows/forum/all/microsoft-phone-link-click-to-call/996a0e46-cd10-42fa-83fa-58a1dcd287f4 your chosen application should function with Tel links.

I installed Skype, set it up in Windows default apps to be the TEL: handler for windows.

The next time I clicked a number in Thunderbird it offered to use that app and set it as default.

Thank you for the prompt and detailed response. By referring to your screen shots, I was easily able to configure Phone Link as the default app and it now works perfectly.

When using Thunderbird for the first time, clicking on a phone number opens File Explorer in the Profiles directory. It would be more helpful to new users if it opened the appropriate Settings window instead.

I often need to copy phone numbers for pasting into documents. I now realise that this is easily accomplished by opening the Edit window and selecting the text. It would be even easier, however, if a copy button was added to the main contact display.

I hope that the above comments do not appear critical. I am very impressed with Thunderbird and have already decided to adopt it as a replacement for Outlook.

Personally I would think it would be helpful if the installer for your chosen Microsoft product registered itself as the default handler for such links, or asked you if you would like it to happen.

It opens in the file explorer so you can select an application to handle such links. The fact that windows hides applications that are installed does not help the process at all, but the option is the lowest common denominator.