Make Teams call without assigned number

nc-novelt With Microsoft Calling plans will need to have a number assigned, but you can then use a CallerID Policy to replace that, for example with a number for a reception or contact centre if you don't want the direct number shown. Manage caller ID policies in Microsoft Teams - Microsoft Teams | Microsoft Docs

If you are using an SBC then no you don't need an externally routable number, my employer gives a core number with an 'extension' number. I don't care as no one has ever called me on it, I cant quite imagine why they would.