Although my symptoms were exactly the same, the cause was most likely not. I say that because I have no idea what a 'line display' is. The day before I started having no audio on answered calls I had two calls do something I'd never seen happen before.

The phone rang, two rings and I answered it, dead air. I hung up, it rang again and the call completed fine. Caller said they called only once, the phone rang at least eight times before I answered.

That said, everything seems to be working fine today.