Apple today updated its iMessage troubleshooting support document to add a scenario that can occur in iOS 26. If you don't activate iMessage or an eSIM when setting up a new iPhone with ‌iOS 26‌, it can prevent iMessage from working.

Apple says that iMessage does not activate automatically when setting up an eSIM in the Settings app after the initial ‌iPhone‌ setup period, so it needs to be done manually.

After activating an eSIM, users can open up the Settings app, tap on Apps, choose Messages, and then turn iMessage off and back on to get the feature to work as intended.

When iMessage is not properly activated after setting up an eSIM, users can receive a Not Delivered alert, have their messages sent as SMS/RCS messages in green bubbles, or see their iMessages delivered from an email address instead of a phone number.

Source: Macrumors