Now at least one caller said that there was no ring at all when he called my GrandCentral number.
Updated: I can call myself just fine.
Updated: Ok, I asked the guy who called me this morning and did not hear a ring to call again. He did, was announced by the GC robot as "unknown," even though he is in my GC address book, and I picked up. He says he heard nothing--not a ring nor an announcement from the GC robot.