The Cellphone is a device which the Angels reverse-engineered (stolen), from the future.
Remmy gave it to Cyril in comic #28 to keep him posted on the situation with Maxx. It appears again in comic #144, when Cyril phones Remmy to tell him about what has happened.
Because Cyril is the only human character to have the Cellphone, it can be considered his signature item .
An item called cell phone appers in Terraria, but it looks totally different than cell phone in LoM. Thats because cell phone appeared in LoM before it was added in Terraria.