IPhone 4...CONTRACT....HELP!!?

okk..i'v got my relatives to buy me n iPhone 4 from USA...wel its was locked version(to at&t with 2yrs contract) n i got it jail-broken n unlocked...now my ques is whether if they get the monthly bill..wil they have 2 pay for that...n if they don't wil any legal action be taken or something...n if m completely not using it...shpuld the bill come anyway???...PLEASE HELP!!