Why are all my messages coming back saying SMS failed to send?

Hello,

If the issue is only with SMS messages, you'll need to also reach out to your carrier. Those details can be found here: What is the difference between iMessage and SMS/MMS? - Apple Support

To use SMS/MMS on an iPhone, you need a text-messaging plan. Contact your wireless carrier for more information.

There are also some great steps found in this guide for when you can't send or receive messages: If you can't send or receive messages on your iPhone or iPad - Apple Support

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Take care!