iMessage not activating with my number

You may need to ensure your iPhone has the newest available compatible iOS version installed.

You can check in Settings > General > Software Updates to see whether an update is available.

(Reminder: It is wise to back-up iPhone first, if you decide to update the iOS version.)

See whether Apple support's online article (link below) can help:

If you can’t turn on or sign in to iMessage or FaceTime on your iPhone - Apple Support

Eg says:

"To use either iMessage or FaceTime, you need to activate them on your iPhone.

If you see an error message when you try to activate, follow these steps. "

When activating iMessage or FaceTime, you might see one of these messages:

• Waiting for activation
• Activation unsuccessful
• An error occurred during activation
• Could not sign in, please check your network connection
• Unable to contact the iMessage server. Try again.

What you need before you sign in

• Make sure that you’re connected to a cellular data or Wi-Fi network.
• Make sure that your device has the latest version of iOS.
• Make sure that your time zone is set correctly. In the Settings app, tap General. Then tap Date & Time.

If you're using an iPhone, you need SMS messaging to activate your phone number with iMessage and FaceTime. Depending on your carrier, you might be charged for this SMS.

If a prompt appears stating "Your network provider may charge for SMS messages used to activate iMessage and FaceTime", tap "Turn On" to allow possible SMS charges.

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If needed: See that linked article at the top of this reply, for more trouble-shooting steps to follow.

Messages Support website : (info. / support / contact options etc.)

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