Connecting Xfinity modem (telephone jack) to wired home network

I recently had an electrician run a cat5 line from my office to the CenturyLink Network Interface Box.  When CenturyLink service was discontinued, I removed the CenturyLink connections from the user side of the NIB and connected the new cat5 wires.  They are in exactly the same place as the old CenturyLink wires were.  I checked for voltage on the cat5 RJ11 outlet and found it to be 51v, so there is definitely connectivity.  However, none of the telephones on the house wiring work.  If I connect the Xfinity modem (RJ11 jack) to the cat5 RJ11, it kills the modem and it takes it a few minutes to reset.  If I plug the modem into another house wired network RJ11, nothing at all happens.  I am puzzled by this.  Any suggestions?  I could use some education on how the NIB works.  Also, I am somewhat puzzled by the fact that there is still voltage at the NIB running through my new cat5 line.  I would have thought all that would have been killed when CenturyLink service was discontinued.