In a statement to 9to5Mac, Apple acknowledged that some iPhone 18 Pro Max users on the AT&T network have lost service and can’t make calls. Chance Miller quotes Apple as saying:
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.
According to Apple, affected iPhones will need a hardware replacement, which is alarming—it’s hard to imagine what could cause a hardware failure like this. The problem likely relates to the iPhone 18 Pro Max relying on a Qualcomm modem, while the iPhone 18 Pro uses Apple’s new C2 wireless chip. However, Apple says installing iOS 27.0.1 and a carrier settings update should prevent the problem from occurring in the first place. The carrier settings update will install automatically within the next 14 days, though you can get it sooner by going to Settings > General > About. It’s unclear whether the problem affects only direct AT&T subscribers or also iPhone 18 Pro Max users on MVNOs that use AT&T’s network, such as US Mobile’s Dark Star and Cricket Wireless.
I read somewhere yesterday speculating that it was caused by an unintended cranking up of power to the antenna when trying to connect to a 5G band that only AT&T uses until something was damaged by the excessive heat. The carrier settings for AT&T fixed the issue by turning off support for AT&T 5G standalone, but once the hardware was damaged, software couldn’t fix it.
Hopefully, they just reduced the maximum power used on those bands, and didn’t disable 5G Standalone altogether.
Most cell networks these days run a “hybrid” network, with a 4G network core that interfaces with both 4G and 5G radios. Your phone uses the 4G protocols to signal and establish 5G radio connections. This works and is very popular, because service providers don’t need to run two core networks at the same time. But it means they can’t provide 5G features that require 5G core-network servers, resulting in what is effectively just a higher-speed 4G network.
In order to provide the newer 5G features, a network operator needs to run 5G core network servers and customer phones need to use the 5G protocols to signal connections. Which means the operator needs to run two core networks - one for 5G connections and one for 4G connections. This is the “standalone” 5G system, where 5G connections use only 5G protocols end-to-end.
Network operators are gradually installing 5G core network servers in order to offer services that require 5G standalone. And, presumably, as customers migrate to 5G standalone, they can scale-down the 4G core network (and upgrade the servers for 5G), to align with actual customer usage.