Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

So, we have no confirmation about Roberts' phone call from the alleged recipient of the call, Winston Lawson. And we know that Roberts lied about the speed of the vehicles and the distance between them. 

So, with no confirmation from anybody, how can we be sure that Roberts was talking to Lawson?

Maybe he was talking to someone else at this point and then afterwards he called Lawson.