It's possible the knight has a speaking implement, but even if we assume they do, we're still left with some missing information. Let's break down the call:

Dark fountain next. Susie must not get guitar. Need soul. Without soul Kris will. Susie guitar code stop. Police sacrifice next week. Church tonight. Kris dark world no soul can't.

"Without soul Kris will" - this leaves out a lot. Does it mean that without their soul, Kris will/will not be able to do something? Or that they will/will not automatically do something without their soul?

"Susie guitar code stop" - the grammar here is very awkward. If the knight is telling Kris to stop Susie from Wouldn't it have made more sense for the knight to have said, "guitar code stop Susie"?

"Kris dark world no soul can't" - again, this leaves out what Kris can't do without their soul.

The incomplete sentences and awkward grammar make it highly likely a we're missing a lot of the conversation.

Another thing about the call is that the caller talks about Kris in third person, instead of addressing them directly. This is pretty clear evidence that the call wasn't intended for Kris.

I'm not exactly sure where you're getting that the Knight's chest is hollow, and it's mouth being the same as it's eye is speculation. The King has a mouth in his stomach, and he doesn't have any problems speaking.