www.alphadictionary.com

I don’t know if this belongs here, or on this forum at all.
Howsomeverwise . . . .

I’ve seen our Western Arabic numerals used with all kinds (possibly literally ) of writing systems, Arabic, Chinese, other Asian, and lots of others. (Notices from Kaiser include contact information in lots of different languages, quite a few with unique writing systems.) I know about a few, but I assume they all have their own numerals.
And it’s not just phone numbers, which use is understandable as precluding confusion.

So (at long last) why come dat?

I can't say why such things occur, but I can say that, as far as my experience with philately goes, it's not always the case. For stamps that are used internationally, yes, but things like Egyptian postage due stamps have only Egyptian numerals on them. No one else needs to know, so why bother?