You've learned Korean, made Korean friends, and they're inviting you out to have some fun.
You can say things like "I'm in! Sounds good! Great idea! Count me in!" etc, but what is up will all these people yelling "CALL!"
콜 - Call!
This is like saying "me!" or "i'm in!"
I don't really know the origin of its usage, but yeah, it's used often!
"How about ddeokbokki for dinner?"
"Who wants to see a movie tonight?"