Apple released iOS 27.0.1 following the initial iOS 27 launch earlier in September. The update fixes an issue that caused the newly launched iPhone 18 Pro and iPhone 18 Pro Max to reboot after a failed Face ID authentication. Or at least that's what Apple has said about it.
However, AT&T is now apparently sending its customers an automated text message urging them to update to iOS 27.0.1 immediately, because it also fixes an issue that leaves an iPhone 18 Pro Max connected to the carrier's network unable to complete calls/text/data, including to 911.
That message from AT&T may have just been wishful thinking on the carrier's part, though, as multiple early reports from AT&T users indicate that iOS 27.0.1 does not in fact fix the problem with the iPhone 18 Pro Max connected to AT&T's network. Those who are affected can't connect to the network at all for calls, texts, and data, and they see "SOS" in their iPhone 18 Pro Max's status bar. This problem is not affecting every iPhone 18 Pro Max on AT&T, but it does appear to be quite widespread.
If you're wondering why only the iPhone 18 Pro Max would have this issue when the iPhone 18 Pro doesn't, that may be because the Pro Max uses a Qualcomm modem while the Pro uses Apple's own C2 modem. Hopefully AT&T can work with Apple to actually get the issue fixed as soon as possible.
Source: GSMArena · Summarized by HeadlinesBriefing