modal verbs - "should" could be inverted in affirmative or some type of condition sentences

1. Should anyone phone, tell him I'm busy.


2. If anyone phones, tell him I'm busy.

I am wondering what is the difference between those?

What is more, I am wondering as to the inversion in the first sentence has happened. Would you please show me a site so that I could study more about such a specific inversion?

Answer

If anyone calls, tell him I am busy.

If anyone should call, tell him I am busy.

Now what is the difference between this two?

The first one is a normal conditional sentence, while the second one suggests that the event of calling is unlikely.

Notes from Practical English Usage by Michael Swan -

Should anyone call, tell him I am busy.

We can re-write this sentence as the following -

If anyone should call, tell him I am busy.

Note in the previous case Should anyone call...., we don't use calls.

Please visit this link for more information and examples of should used at the beginning of the sentence with if dropped, as you quoted in your question.

N.B Pam Peters in Cambridge Guide to English Usage suggests that the usage of should in conditional sentence is becoming less often. Conditional uses of should with if included in sentence now sound rather lofty, but the inverted should at the beginning of the sentence with no if included in that clause is still a neat way of prefacing a condition.