Switched carriers. Now unable to send text messages to non-Apple devices

Welcome to the Apple Support Communities.

Based on your screenshot, reading If you can’t turn on or sign in to iMessage or FaceTime - Apple Support may help for iMessage. But, for SMS issues, you’ll need to contact your carrier: Wireless carrier support and features for iPhone in the United States and Canada - Apple Support & Contact a third-party vendor - Apple Support.

Jack

colleenellenwood wrote:

Switched carriers. Now unable to send text messages to non-Apple devices.

That is the exact reason why you are not able to activate iMessage. (iPhone to iPhone)

To activate iMessage and FaceTime using your Phone number

First, check to know whether you have an SMS plan enabled at least for One SMS with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS Only)

If you can't turn on or sign in to iMessage or FaceTime - Apple ...

If the above process does not work then simply disable WiFi and enable iMessage over 4G

Types of messages?

• iMessage? --> Check your internet stability and connectivity. Free message servcie between Apple Devices. If the Apple server is the issue, it will show here --> Support - System Status - Apple
• SMS? --> Does not need internet. DO you have SIM with SMS enabled in it? If yes, contact your Carrier Service Provider for network coverage and outages ->: Down Detector - outages