Teams and low-hanging fruit
Reminds me of Skype, back in the day. Before it was adopted massively by companies everywhere, you were on your own when you installed Skype. So it was normal to be able to be contacted by anyone, since when you start out, there's not really any way to get to to know anyone.
But when companies finally got a "professional" version, existing employees were automatically (from the employees point of view at least) added to the contact list, so there was no reason to allow just anyone to contact you any more. Not for the majority of employees anyway (Marketing is always an exception).
With Teams, I expect the situation to be the same. You work in a company that has Teams for Suckers Business, you have all your company colleagues available, why should you accept comms from outside ? I suspect the only reason is for people to keep in touch with their personal acquaitances, which opens the nice big barn door allowing this kind of shenanigans to happen.
Besides, if you work in a company and you don't call IT for your problems, you are the problem.
Either that, or company IT completely sucks, in which case you urgently need to start looking elsewhere because, if company IT sucks, the company will not survive for long.
I am lucky in that I do not have Teams (or any similar product) nor do I need it. If, however, I was saddled with a customer who had that installed by default on the computer I was to use for work, I would find the way to restrict comms to internal contacts only, then I would try to keep it out of my way.
There will be no answering somebody I don't know, and especially not for IT or security issues.