the act of taking your friends phone, dialing their ex-girlfriend/boyfriend's number, then tossing it to them as it rings. Usually results in the "victim" of the grenade having a very awkward, yet hilarious exchange with their ex.
Me: Hey Nick! Catch!'
Nick: DAMNIT!...I mean...um hey Julie
Julie: Why the hell are you calling me?
Nick: Sorry I got phone grenaded.