iMessage phone number spinning

Starting today, my iPhone phone number is no longer registered as able to send & receive iMessages. If I look in Settings > Apps > Messages > Send & Receive, my phone number has a spinning gear icon next to it, which according to Apple support articles I've read means that iMessage is attempting to activate the number but is unable to verify it with the carrier. I'm still able to send & receive iMessages using my email address as the identifier.

The number works fine for calls and SMS. It's only iMessage that's having an issue. It has always worked fine before and my number was successfully registered; this only started today. I haven't made any changes other than to update my iPhone to the latest iOS 18.2 this morning (I don't know if that's related).

I've tried turning iMessage and FaceTime OFF, restarting my iPhone, and turning them back ON as recommended in the Apple support articles, but the problem persists.

Thanks.