If your iPhone 18 Pro Max always says SOS, you need to get it replaced. Since launch, iPhone 18 Pro Max models have been dropping off the AT&T network. In some cases, never to return. After an OS update earlier in the week, and a carrier settings update on Friday, the problem may no longer manifest for those who haven't seen it. But, that doesn't help if your iPhone has issues already.
Apple has given instructions on what to do. "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."
Apple also says that already-impacted devices need to be replaced. The best way to do this is at an Apple Store, but some AT&T corporate stores may have stock for replacement as well. AT&T says the carrier settings update will auto-populate in the next two weeks. However, we don't recommend waiting.
Source: AppleInsider · Summarized by HeadlinesBriefing