I can't use my iMessage using my phone number.

If you get an error when trying to activate iMessage or FaceTime - Apple Support

Add or remove your phone number in Messages or FaceTime - Apple Support

You can follow these steps to see if it helps but do not leave any steps out and if it does not help do what the last line says...

1. On your Mac:

• • In Messages, choose Messages > Preferences, then click Accounts and select your iMessage account. Click Sign Out.
  • In FaceTime, choose FaceTime > Preferences, then click Sign Out.

1. On your iPhone, iPad, and iPod:

• • Go to Settings > Messages > Send & Receive. Tap your Apple ID, then tap Sign Out.
  • Go to Settings > FaceTime. Tap your Apple ID, then tap Sign Out.

1. On your iPhone:

• • Go back to Settings > Messages and make sure that iMessage is turned on.
  • Tap Send & Receive, then choose “Use your Apple ID for iMessage” and sign in with your Apple ID.

1. Using the same Apple ID, sign back in to Messages and FaceTime on your other devices. 

If you still need help, change your Apple ID password and repeat the setup steps. If that doesn't work, contact Apple Support.