Go back to dumb-phone

Drop the iPhone and get a cheap $20 go-phone that uses SIM cards. If you have the newest iPhone with the microSIM, you can find an adaptor online for like $3 or less to make it a normal SIM card. Insert the normal SIM into dumb phone and drop data and reduce all services to bare minimum until you're out of contract. Use iPhone as planner, for games, or other app reasons and connect to Wi-Fi only with no SIM. Dumb phones work better sometimes. First hand experience talking.

Only other SIM card service is from T-Mobile in the states, you can unlock the phone to work over any network by requesting unlock code for free from T-Mo, or vice-versa.

ATT is infamous for overloading their networks and then "fixing" the problem with new rate plans like higher cost for less service in disguise with each iteration.