iPhone "number not available"

I recently made a call from my iPhone. Even as the called number was ringing, the iPhone put up a notification that that the called number was not available. I seem to recall reading about this phenomenon here at TidBITS, but my search skills were inadequate for the task of finding the thread. In the vein of teaching a person to fish, would someone tell me how I should have searched for the thread? Failing that, would someone give me a fish, I mean provide a pointer to the thread? Thank you.

This might be it:

I couldn’t find anything in the cited thread about the iPhone saying a telephone number was unavailable or not accessible. Such a message was (I believe) discussed as a bug here on TidBITS following some iOS update, but I cannot find the topic.