This is a short and sweet theory essentially born from a few odd facts from Deltarune. I’ve termed it COMMUNICATION THEORY.
There’s no reason given in-game, so we’re left with the pretty mundane explanation that “the internet just goes down sometimes.”
When Toriel calls the police she says "Thank goodness, I finally got through..." implying she was having trouble getting through for some reason. Why would she have trouble getting through to the police? It’s true they might be investigating Kris taking everything from the Computer Lab, but it doesn’t feel like that would cause such a disturbance.
While in the Dark World, phones are blocked and instead just play the iconic garbage noise. It should be noted as well that phones don’t work in the presence of Gaster Followers in Undertale either. This might be related - or it might not be.
This theory proposes that all these events have a root cause. Darkness disrupts communication, particularly of the technological kind, such as phones. This is why Kris’s phone doesn’t work in the Dark World. As the Knight opens up more fountains, the increasing amount of Darkness in Hometown is slowly disrupting communication in the Light World as well, causing the internet to go down and phones to become unreliable.
Storywise, this means in future chapters we’ll see phones start to break down, and potentially other forms of communication like television. This might even serve as a convenient reason to not get help from outside Hometown if knowledge of Dark Worlds and the Roaring becomes public.
It could also have thematic relevance. The idea of fiction and stories seems highly important to Deltarune. Cutting off people’s method of sharing fiction could be thematically impactful.