how do i find out my uk network provider on my iphone 4s?

i brought the phone second hand while back n just forgot about it as i already had a iphone 4 set up, iv recently sold the 4 and now want to get the 4s up n running, but i updated thru itunes n its put me back at the activation set up, i have no way of contacting the sellers n dont have a clue what network its on, is there anyway of apple checking using the imei number or anything similar?