On some cell phones it comes up as a flat line instead of bars or an x.
May be used as Flatline, Flatlined, or Flatlining to describe having no cell reception.
Dude 1: Dude, Where is this party?
Dude 2: Idk, call Sarah.
Dude 1: I can't I'm Flatlining.
Girl: Why the hell didn't you call me back last night?
Boy: Sorry babe, I had a Flatline.
Dude 1: Anyone have cell reception?
Dude 2: Naw, I'm Flatining.
Dude 3: I'm Flatlined too.
Dude 4: Got a Flatline.