Seriously? You swapped the phone, line cord and KSU and the problem persists? That's not possible.

The fact that the trouble follows 3, 6 and 9 suggests either a problem with the set membrane (the digits are in the same column) or some bizarre touchtone detector problem with 1477 Hz. Which begs the question: Are you also having a problem with the "#" key?

Of course, the business of so-called unlucky x13 and chicken bones may need some reconsideration.

"Press play and record at the same time" -- Tim Alberstein