A sim card is a physical device that AT&T has a 1:1 reference to a line of service.
Originally Posted by Viper5dn
a line of service is a phone number for most people. or it can be the service on an internet card, or pager, or whatever. From a comparative land line standpoint Dry loop DSL does not have a phone line, it has a line of service it uses.
With wireless the plan features go on the line of service. txt, data, voice, whatever.
Again comparing to a landline, Uverse, DSL and phone service all shares the same line of service. they're roughly comparable to the features on a cell phone line of service.
Whereas with DSL you get a physical box that works with the signals and the phone company has rules that let you connect that box (i.e. username/password is required) a cell phone sends down device codes. each phone is programmed for each feature that may be enabled or disabled. like when MMS was added to the iphone the phone needed the programming added. a device code enabled this feature and a matching entry on your plan let's it be billed right
they can even scan their system for the IMEI and see if it matches what's on your plan. It takes some hacking by a rep to get their system to see otherwise. It's hard to be sure if they scan the phone or the IMEI in their records, I bet both.
if you have the wrong plan for the device billing may see device X, but to get unlimited data you need plan Y and it sees plan Z and it bills accordingly.
this is why when moving between devices you have to make sure the plan matches the device it's in. if you don't, you're risking a huge bill.