Hi @rybread168 ,
You'll have to check the cabling between the downstairs and upstairs sockets to ensure that there isn't a broken connection on either L1 socket connector cable termination point.
If that appears to be OK then it could be a faulty line socket connector that the phone plugs into upstairs or perhaps a fractured wire in the cable connecting the two sockets. It could even be that the cable has been chewed at by a rodent. They simply "luv" plastic covered cables.
If no problem is found at either of the sockets, you'll have to test that the cable between the two sockets is OK. Identify and disconnect the L1 cable wires which go from the socket downstairs to the socket upstairs. You can leave the "street cable" wires connected to the L1 socket, presumably this is downstairs. Disconnect the L1 wires from the socket upstairs. They should be the same colours as downstairs unless there has been some other wiring arrangement made.
Twist the bared ends of the wires together at one end of the internal cable, upstairs or downstairs, and then connect an Ohmmeter to the wires at the other end of the cable to test for continuity.
If there is no continuity then there is a break in one wire at least. it may be both if the cable has been chewed.
If there are spare wires in the cable between the two sockets you can use another wire to replace the faulty one. Of course you'll have to identify which one was faulty first. if there are no spare wires in the cable, then a new cable has to be run in.