iMessage not listed in cellular settings iphone 8 ios 15.7.11

Thank you: To send an SMS message, you need cellular network connection.

Check cellular is turned on in in Settings > Cellular

Did you check Settings > Cellular > Cellular Data (turn on so you check the list of apps)

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Also check Settings > Apps > Messages >  Send & Receive

Check if you have selected phone number(s), or only email address(es)

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If needed: Apple have a Messages Support website online:

There is a "Get Support - start now" link at the very bottom (online chat / phone / email etc.)

Messages for iPhone, iPad, Apple Watch, and Mac - Official Apple Support

Re: iMessage not listed in cellular settings iphone 8 ios 15.7.11

If you haven't yet installed it:

About iOS 16 Updates - Apple Support says: iOS 16.7.11

and "This update provides important security fixes and is recommended for all users".

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Try cellular data settings (if your provider's plan allows cellular data use (charges may apply)

For iOS 16: View or change cellular data settings on iPhone - Apple Support

Eg: says:

"Set cellular data use for apps and services

Go to Settings  > Cellular, then turn Cellular Data on or off for any app (such as Maps) or service (such as Wi-Fi Assist) that can use cellular data.

If a setting is off, iPhone uses only Wi-Fi for that service.

Note: Wi-Fi Assist is on by default. If Wi-Fi connectivity is poor, Wi-Fi Assist automatically switches to cellular data to boost the signal. Because you stay connected to the internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data, which may incur additional charges depending on your data plan. See the Apple Support article About Wi-Fi Assist. "