Cox is backfeeding dialtone from their cable modem into a nearby jack. The wiring from this jack goes back to the existing NID where all of the other station wires are terminated. I'm sure they disconnected the original copper service drop inside the NID (or simply removed the test plug) and left the station wires bridged together. This is how dial tone is probably working on all of your jacks.

When you installed the DSL service, the technician would have to reconnect the copper service drop and connect that to one of the station wires that goes to where the modem is located. I'm sure this un-did the bridge that Cox had in place. But that is why you probably didn't have dial tone on that jack when DSL was installed.

It would have been easy enough to leave all of the R-G pairs of the station wires "briged" in the NID for dial tone, and then use the B-Y of the station wire for DSL from the copper drop. You simply change out the jack for a dual service jack.