iMessage not signing in after backup restore

Hi.

My iMessage won't sign in after I did a backup restore of my iPhone 13.

Have turned it off and on in settings but still nothing.

I'm getting banner notifications but as soon as I open the Messages it says:

"An error occured and your account (Apple ID) is signed out of iMessage. You may miss messages as a result. Please go to Settings to address this issue."

In Settings the iMessage slider is on but it says: "Activation unsuccessful. Turn on iMessage to try again."

Has anyone seen this before?

Thanks.