When they tell you that they are going to call you back, here is what you do. Tell them, "That sounds great. If I don't hear back from you by next week, I might try catch up with you sometime. Is that okay?"
THAT'S IT! The prospect almost always agrees and it is a perfect solution. It give the customer time to keep their commitment and it lets them feel in control. In reality, YOU maintain control because you now have their permission to contact them if they flake on you. Its a WIN/WIN!