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After unplugging the phone jack in the alarm system box, once again, for the 6th Thursday in a row, at 1:22am, my phone made an outgoing call to 855-294-2278. I guess I'll make another call to Cox. Maybe whoever I reach will have something new to suggest, other than switching out the modem. Thanks to all for the ideas I've gotten from the replies to my original post.
Fax machine...call blocking software...computers left on all night...router left on all night...TiVo...DVR. There's something you connected 6 weeks ago. WiFi-enable refrigerator? Check the Network Interface Device/Unit outside you house. It has a phone jack.