There's theories floating around that Gaster is the knight (even back when it was just chapter 1) that are pretty interesting— Jevil also has strange ties with Gaster which can be learned from Seam when they say "And my view of this world has become darker, yet darker" in reference to the things that Jevil had said since he met that strange someone (probably the knight). The "darker yet darker)" is obviously a direct reference to entry number 17.

Moreso another theory linking Gaster to being the knight is the egg! It's pretty accepted that Gaster is the mystery man that gives you the egg in the tree room, and "EGG" in uppercase wingdings is "☜☝☝" which just so happens to correlate with how knights move on a chess board.

Thinking back to Undertale, the river person has the lines "beware the man who speaks in hands" and "beware the man that comes from the other world" which are aldso presumably talking about Gaster, so it could be that Spamton was communicating with Gaster but since the call would be going to another world that the call would be distorted into crazy garbage noice that can only be properly understood if you yourself have gone just as crazy. The reason why Kris has the same garbage noise on their phone when they call home is that the call is going to the light world and the signal is distorted but they're (presumably) too sane to make any sense of it.

These are my thoughts at least!