'Who.....' 'Whom' is extremely formal so is very unnatural in this context. It is mostly used with a preposition before it, e.g. 'For whom is the money?'. However, this is still very formal.

I'm afraid that all of the answers so far are wrong. "Whom" is not informal. Who/whom are used as subject/object, just like he/him. So, if you would say, "Are you calling him?" you would say "whom are you calling?" Similarly, since "he called me" is proper, so too is "who called me?" Saying "who are you calling" is technically wrong, because it is equivalent to saying, "are you calling he?" That all being said, I said that "who did you call" is technically incorrect because the use of "whom" has become quite rare, and even can sound pretentious nowadays. Most people say "who are you calling," even though it is technically incorrect.

Who are you phoning? Who is the money for?

who are you phoning ? who is the money for ? or you can say for whom is the money ?