An iPhone 18 Pro Max that has already lost service on AT&T won't get it back from a software update. Apple has now confirmed the problem, and 9to5Mac reports that Apple says those phones need a hardware replacement.
Apple's statement is about stopping it from happening: "We have identified an issue affecting a small number of iPhone 18 Pro Max users on the AT&T network that may cause a device to lose service and be unable to make calls. We released iOS 27.0.1 earlier this week and strongly encourage all iPhone 18 Pro Max users to update now, and are also issuing a carrier settings update today. These updates together are meant to help prevent this issue from occurring."
Phones that are already cut off are a separate case. Apple says the updates won't restore service on them. Their owners can contact Apple Support or AT&T, or walk into an Apple Store or an AT&T location.
Owners had been saying iOS 27.0.1 didn't help them. AT&T texted customers to install it immediately because some phones might be unable to complete calls, texts or data, including to 911, and the carrier settings update, version 72.1, arrived on Friday along with a requirement to install 27.0.1 before setting up the phone.
Apple says a small number of phones are affected, while MacRumors reported that thousands of owners couldn't connect to AT&T for calls, texts and data. Apple's statement doesn't give a count, and it doesn't say what causes the failure.
The carrier settings update downloads automatically over the next 14 days. To pull it sooner, open Settings, choose General, then About.