My call log shows 7 calls from this number in 3 minutes today. Yesterday a total of 7 unknown callers altogether, and on 3/28 4 calls from 251-662-9686 in 2 minutes.
My guess is a new number is used every day. Total has been 31 calls in the last week from unknown numbers. This is out of control.
If you're answering these calls, manually or automatically, some of them may be passed along that quickly. It certainly can get out of control with receivers' answering. You basically have to let your phone ring clear through with unknown numbers and give it time before you may see a decrease. A call blocker could be of assistance if you're unable to completely miss calls.