all numerals so there's no ambiguity nor is there any other digit for which the oh might be confused.

Back when I was learning German, I was taught that when giving someone a phone number or other series of numbers, one should pronounce "zwei" (2) as "zwo" to avoid confusion with "drei" (3). Is this really done, or is it some old-fashioned relic, or some overly-specific affectation (like spelling cat Charlie-Alpha-Tango)?