Hellpp me plzzz?

I whant to by a blackberry curve fome ebay or at@t or ciguler. But i am on a contarct for 2 mor years with US Celluler. And i do not like the phone that i have. Can I get it and use it on there plane. I am on the famley plane if that helps at all. the botom line is can i by a phone fome some on els and use it with them in stead of bying one of thare blackberrys that thay have. thay do not carry the blackberry curve. The onley ones that do it at@t and singuler that have it. Just can i by fome them and use it thare. just tell that you can and can you tell me if you blackberry. And with the black berry can i go on yahoo.com and anwers with it on the go:) And i would get the i phone but i do not like the lay out on it and i have a pc and not a mac. The stuff the i do i would have to chang a lot of the pc that i have. I like the stuff that mac has it's just over pric a lot of there stuff is and i do know mac mom and dad and my grand ma and pa have one. so if you have a mac it ok