AT&T Updates

Wait a minute... If you're wanting to download the AT&T Marshmallow update, don't you need your AT&T sim card to get it from the AT&T servers? Without the sim card, how does the phone know where it's downloading the update from?
 
In the past, no you don't have to have sim card because the ATT update app knows where to download OTA update. But recent years, things have changed partly because updates are much larger downloads. Now you have to have an active ATT sim card in the phone and connected to ATT network in US in order to see the updates. The actual download can still go through wifi.