Monthly charge on unlocked smartphone,if ur not using internet

Obviously, I'm not tech savvy, I was given a smart phone, 3 g I-phone,Do not have the funds to pay an extra fee for internet charges, so I would just be using it as phone no data, I was told even if it is jailbroke(?) that my phone company would know immediately as soon as I put my sim in, and there would be an automatic monthly charge because it is a smart phone (even if i didn't use internet or data. But was told if I change phone companys, like Tmobile i could use it with no extra data charges. Anybody know what im referring to? Thanks in Advance, jenniferL