So Carlo beats the crap out of Connie and shows how clever he is by pretending to leave the apartment, but is really hiding behind a curtain. This is how he knows that she caled Sonny. Once she hangs up she probably says to herself, "while I am staying here waiting for Sonny to come, I guess I'll clean up this mess. Then Carlo slinks out, calls Barzini to tip him Sonny is on the way and Barzini then has about 10 minutes to tip off the guards at the toll booth and get al his hitters in lace to shoot Sonny. Not plausibe timing,but the real question is did Carlo go off to meet the woman who called COnnie to cancel the date, and when he found out he had been stood up what did he do to HER?