I did not call AT&T & I like Switching back & forth from iPhone 5 to nexus 4 a lot don't no if this will work with galaxy s3 maybe someone else can confirm that has gs3. Got these APN from another forum
I have an iPhone5 and just got my Nexus 4. Got things working using a nano to micro sim adapter and this APN:
Name: (whatever you want it to be)
APN: phone
Proxy:
Port:
Username:
Password:
Server:
MMSC:
http://mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
MMS port: 80
MCC: 310
MNC: 410
Authentication type:
APN type: default,mms,supl
I rebooted after entering and activating the new APN...might not have to though.