Problem with the eSIM
According to 9-5 Mac, this problem only affects the eSIM cards. If you forget your SIM card, a SIM card allows you to use your phone to send and receive text messages.
The purpose of the eSIM chip is to make setting up an iPhone easier. This bug only affects iPhones with iOS 15.4, 15.5, 15.6 beta 1, or 15.5.
T-Mobile and Verizon seem to have known about the bug. Gurman was among those who reached out to their carriers for help. Apple has yet to release a patch or acknowledge the bug.
Solutions
Gurman suggests inserting a physical SIM card into your iPhone to prevent the bug.
You may also have your eSIM removed and a new one issued. Gurman cautions against this because it is too difficult for most people.