Intercom not working on a single extension

Intercom feature is no longer working on a single extension. Using Cisco spa504G phones. Can intercom all extensions except one. Also can not page through this phone. If I setup a new page group with this phone only in the group and set busy extensions to Force, I can then page to this phone.

I have done a factory reset on the phone. No change. I am manually configuring this phone, No provisioning file to worry about.

I have dialed the intercom allow code to be sure it was not accidentally de-activated.

I think I can remove the extension and rebuild to resolve, but would like to know what has caused this issue in the first place unless it happens again.

Are you sure the phone is not in DND mode?

Yes. Can call the extension, and it rings. Just no intercom. *80xxx.