What is the difference between "who is this?" and "who is calling?" when we answer to the telephone?
Is the second one more polite than the first one?
Lastly, is there any difference between AmE and BrE?
Who's calling? is a neutral way of asking the caller to identify themselves.
May I ask who's calling? is more polite.
Who's this?, depending on the tone of voice used, could sound terse and unfriendly, suspicious, impolite, angry at being disturbed, and so forth.