Calls going VM after one ring, even though DND is off

All of a sudden, my calls are ringing once and then going to VM, and Do Not Disturb is turned off. If I'm sitting at my desk, my iMac will ring once also and then it shows a missed call. I've switched DND on then off, restarted the phone, nothing seems to help. I've tested and called from my office phone, rings once then goes to VM, if I hang up and call a 2nd time, then the call rings right through. What is going on?