The easiest/sure fire way to update with local towers is to turn your phone completely off for at least 1 full minute, and power it back on and the radio inside should search for the local towers automatically.
I worked for at&t both english and spanish customers in wirless and uverse and other home services as a customer care rep by phone means call center also work for tracfone net10 and straight talk also got some knowlegde on verizon. also work in sales for direct tv and uverse hsi and other att products
my tip : remember when dealing with att reps and managers to insist in your issue to be resolve but always keep in together yourself depending on representative and managers jugdement the faster you can get things done, I know its bore and it can be frustrating having to repeat yourself many times but the more you do it the better so the agent wont forget whats your issue about or an important thing that might be the key to solve it its there in your story. remember that people in call centers gets 100's of calls per day per agent. some of the reps are really good some others are just lazy make sure the one you got sounds secure if not ask for a different person.remember to be patiente they have to verify everything before answer even if they already know the answer the have to verify in their systems for quality purposes.
hi the phone its supposed to do that automaticly but yes you can shut it off and then back on after a cuple of minutes you can also check out att device tutorials here select apple then your device iphone 4 then you will get the whole list of tuturials and how to here... http://www.att.com/esupport/main.jsp?cv=820