an idiot of a salesman

[Kotfor]

There is such a phrase.

Is that idiot of a salesman calling again?

Will the meaning be changed if we say

Is that salesman of an idiot calling again?

[Chicken Sandwich]

NOT A TEACHER

The second one doesn't make sense to me.

As you said, the first one is idiomatic: an idiot of a salesman, an idiot of a lawyer, an idiot of a doctor, and so on.

[Kotfor]

My fuzzy logic.

A man of importance = 1) First, he is a man 2) he becomes important. (not vice versa )

An idiot of a salesman = 1) First, he becomes an idiot 2) and only after he becomes an idiot he may become a salesman.

I understand that this may not look persuasive, but may be there is something in it. Anyway, I get it.

[Chicken Sandwich]

My fuzzy logic.

An idiot of a salesman = 1) First, he becomes an idiot 2) and only after he becomes an idiot he may become a salesman.

NOT A TEACHER

No, it doesn't work that way. "An idiot of a salesman" just means that the salesman is an idiot. It doesn't mean that s/he was an idiot prior to becoming a salesman. It's possible that s/he was an idiot prior to becoming a salesman, but that's not the intended meaning of the phrase.

P.S. More precisely, as emsr2d2 said below, s/he is an idiotic salesman. Of course, it's possible that the "idiot of a salesman" is not an idiot when it comes to other aspects of life.

[emsr2d2]

Effectively, it means that he's an idiotic salesman.

[Kotfor]

Are there any similar phrases without the word idiot though?

[Barb_D]

And that fool of a doctor never thought to ask her what other medications she was taking.

[Chicken Sandwich]

NOT A TEACHER

"moron of a doctor" also works. Other synonyms of "idiot" can also be used.

[MartinEnglish]

It's usually pejorative:
That bitch of a nurse said that...
That fool of a manager told me that...
In my last job I had a wanker of a boss who..

etc ad nauseum
I can't think of any positive versions! It's an interesting point, never thought about it before.

[Barb_D]

That peach of a girl
That sweetheart of a nurse
The gem of a teacher

It works both ways.