What leads Susan Carol to believe Professor Whiting is speaking with Steve Jurgenson in Chapter 15?

While waiting for Brill in his hotel lobby to go on his interview with Feeley, Stevie and Susan Carol hear a familiar voice, which they identify as Whiting's, and they see him walking with another man. They suspect the other man is Jurgenson, so Susan Carol asks the hotel to ring Jurgenson's room. She gets his voice mail, so they conclude the man with Whiting could be Jurgenson.