iphone 4 3g problem after change in card sim and company

Recently swiched from Vodafone to Wind due to lower monthly contract and although followed the "reset networks settings" step

when i switch from 3g signal to wifi and then come back to 3g i get a message saying that i am not subscribed to a cellurar service!

Even if i do not switch from 3g to wifi i noticed that after one day of the reset in the network settings i suddenly lose the 3g signal!

Wind customer service tells me that it is the cellphone to blame but i have no reason to believe that since for 3 years on vodafone i had never such a problem!

any ideas about what i can do???