I have the panasonic CA connected to my office extension, by default it is set to turn on absent after 5 min, so when I went a way from my computer for that time I'd look at the phone and the clock was gone. so when I read the fault, like you I tried it and hey presto no clock.
It is funny that it will allow you to turn on a blank message for absent message!!!!!!!!

I should have said I just know everything smile

“I have not failed. I've just found 10,000 ways that won't work.”