My parents pay my phone bill... should i start paying it?

[help with questions]

My parents pay my phone bill.. At&t

i have an iphone.. but unlocked it to use the same plan they have for me..
the mobile to mobile.. free text.. but no internet

so i basically use my iphone like a regular phone with no internet.. but i connect to the internet where i have wi-fi

i want a 3gs though..
and some how my phone updated its own plan? even though its unlocked..
i guess att searched it out or something..
but the point is.. that now i have to call att and take it off the internet off of my plan.. because my parents said so..and they wont let me pay the difference..

and they said "you have to remove the internet off of the plan because it's more expensive for the iphone..either that or pay your own bill"

i work... part time..so an iphone is like.. 100 a month right?

should i just get my own bill and have everything the iphone is suppose to have?
plus with the one i have i cant send pics or messages.. so its totally weak.

i know that paying the bill will boost my credit.. but im feeling worried about the 2 year contract..

helpppppppp!

or suggest