One day a child fell into the underground, being taken in and cared for, everyone loved them as their name was Frisk, the child wore a soft light blue sweater with two white stripes, their eyes always dolled and shining with life as the child knew no harm, walking around the underground but before they knew it the kid wanted to stay with the monster's they had befriended. Understanding the difference between humans and monsters but still wanting to be friends despite all the risks or danger they could be in.
As the days turned into months it wasn't till things turned sour, a earthquake had struck the underground the water was rising and all the monsters could do is run and get away a few tried to help and a few were successful in which a few more had survived but unlike the ones who could see another day, Papyrus and Frisk. Were unable to.
Everyday after that Sans would walk to the same phone booth, it never worked, it had fell in during the earthquake and looked to be to damaged to even be moveable, it was outside the maps definition.
The sounds of the water falling from the ceiling of the underground, the sound sounded like rain, and it acted like it too, as Sans held his umbrella staring at the phone booth because to him.
It was the last hope he had to talk to the void that took his family.
Welcome to Tsuntale.