I have a plan with at&t, but my contract expired a few months ago. My phone is in pretty ugly condition but it still functions. I want a new one--but i want a really cool one this time around. So naturally, I look to Japan. I've found a design that i like (http://www.ebay.com/itm/SHARP-SH01A-SH-01A-Unlock-8MP-GSM-Japanese-Cell-Phone-/270540669474#ht_13228...) scroll down for information on the phone's signal, etc.

I've been assured that it will work with GSM, with the frequencies(? not sure on the technical terms) that AT&T functions on. I know that i need a hypersim card in order for the phone to be "unlocked" and work with AT&T, and i'm planning to buy an itex hypersim, which are rumored to be the best.

My question is, in order to NOT DO ANYTHING ILLEGAL but still have this phone work for At&T, what do I need to do? Will I have to get a new contract (i don't want to if i don't have to)? It isn't a smartphone, but will be there an extra charge? Will I have to check at an at&t store with an employee that it's okay to use a phone that ISN'T AN AT&T PRODUCT with their service? 

Or is it okay if i just buy the phone, switch my current AT&T sim card into it along with the hypersim, and just have a dandy time with my new phone and not even have to contact AT&T? I just want to make sure i'm not doing anything illegal at all, or violating policy.

Thank you! I hope to get a reply soon :/