Hello.
I never solved any problems this kind, but I think I can help you.

I was thinking about it, and this is what I came up with:
If everybody would call the same one person, which is n-1 calls, he would know all the secrets. And then he could call everybody, which is n-1 calls too, and tell them all that he knows.This gives us 2n-2 calls.
I don't know if this is the minimum number of calls, but I did what I could.

I hope you'll find the right answer.
Good Luck!
-Liliya