i’ve been using my at&t network cellphone since quite a long period of time. i need more mordernized model now. is there any place in the states where i can find my choice of at&t cellphones? but i need atleast one month money back gurantee so that if i discover any problems sooner or later within one month, then i can get it exchanged or get my money back. please let me know the details about purchasing any used at&t cellphone.