said by malletto:Plug rj11 cable in there, then the other end of that into "something".
If you buy the type of cable from the eBay link, that cable would have a RJ11 end that goes into your cable modem, and the other end of the cable is RJ45, which goes into your office jack (in the distribution panel).
In your office, you have another RJ45 jack, so use a SECOND cable (same cable from eBay), plug the RJ45 end of that cable into the wall jack, and plug the RJ11 end of that cable into your phone.
