Iphone 3g unlock; T-mobile + Iphone= charges?

[Chrismon71]

if you unlock your iphone and insert a SIM card such as t-mobile, will it charge your t-mobile service extra since you used the data plan on the iphone? or will your t-mobile bill be the same and nothing be charged. Help would be nice, thanks!

 

[melwan]

I think you have to have a data plan wit ht-mobile, otherwise it will charge per kb.