Doesn't seem to play well with the native Microsoft Phone Link. It doesn't allow declaration of Phone Link as the default app for handling the tel: link nor does it allow you to use the system default app.

Numbers have to follow exactly the one single format that is recognized by this Add on, or they won't be linkified. +49 123 456789 is detected, but it's not capable handling numbers with special characters as they are customary in Europe like +49 (0) 123 456789 and +49 123/456789. There's no "phone" or custom protocol to use with.

The former Add on "telify" did all this and worked like a charm. But since Mozilla changed the add on API many many extremely good and helpful add ons are gone without replacement.