This is a more interesting question than it appears to be. ........
How did Barzini figure out this would happen? How was it he had shooters stationed at the toll booth? Just in case?
Not so much interesting in the who made the call but from a perspective of "the war". Barzini may have had numerous plans for getting Sonny and Sonny was cautious when his temper wasn't provoked. When he "visits" Lucy he has a small army with him. As he leaves the building he looks up and down the street keeping an eye out for trouble. Barzini likely had several plans; we don't know about the failed ones. He knew Sonny had a weakness and he exploited it. Sonny threw caution to the wind and paid for it.
