iPhone keeps saying "waiting for activation"

To use iMessage or FaceTime, you need to activate them on your iPhone, iPad, or iPod touch. If you see an error message during activation, 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.

Check your device settings

1. Make sure that you’re connected to a cellular data or Wi-Fi network. 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.
2. Go to Settings > General > Date & Time and make sure that your time zone is set correctly.

4 Step Process --> Turn off and restart iMessage and FaceTime (The order / sequence is very important)

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device.
4. Turn iMessage and FaceTime back on. (Watch the GIF below)