help me find a horror/mystery manga thankies lovelies

i suddenly think of this but i cant remember if it was one manga or two different mangas. horror/kinda thriller(?)/mystery/shounen.

it was of a group of friends, i forgot how many but at least 2 girls and 3 guys. two of them dated each other. the two girls were those opposite archetype: one is big boobs shy scaredy cat, one is kinda tough tsundere. don't quote me on the big boobs and tsundere but you get the gist.

anyways, they perform a ritual every year, because when they were children, they murdered a man and buried his bones in a cave in the mountain. every year, they check for the corpse (or bones in this case). i think one day they lost the bones and panicked. they found a cellphone that told them something idk i forgot. then, they tried to find the culprit who took away the bones before that someone alerted the police.

the stranger in the phone sometimes give them messed up commands. i think one of them was to be naked??? but the one i remember the most is that they were told to dismember a corpse of a man and throw the remnants away. they were told to use cat litter to hide the smell.

i think they did that twice, but the second corpse was headless.

[MAJOR SPOILER]
the headless corpse was actually one of their friends: a boy who was dating one of the girls, and was the handsome guy type. on that second corpse, they were actually separated in two: a girl and a boy dismembering the headless corpse, and a girl and a boy going to a place of meeting to find the boy's head.

also the guy who called them for those weird commands was actually told to do so by the big boobs shy girl, and that she's also the one that murdered the handsome boy. i forgot how exactly.

anyways.

help.