Kramer says his new phone number is 555-3455, or FILK. But Jerry says that he must be getting calls for 555-FILM, which is the same number. But on a standard telephone, 5 represents the letters JKL, and 6 is MNO. So, unless all these people are dialing the wrong number (555-3456), then Kramer's new phone number is incorrect.
Well, he says his new number is 3455, which IS FILK. If someone were dialing 3456, which is FILM, they could possibly hit the 5 a second time accidentally before hitting the 6, which would dial Kramer's new #.