So in the end, this was the culprit: When this was originally set up I chose Tone 5 in 15-08 for the VE 252 ringtone, so the agents could distinguish it from any other call. I have found that if I choose anything higher than Tone 4, it will not audibly ring the set. I don't know why this suddenly stopped working, but at least it works once again. I quickly checked to see if it might be COS related, but didn't find anything.