I really doubt there is an electrical box behind the drywall.
Telephone line coming out of the floor.
The phone line rather like water pipes become your responsibility as soon as they cross the boundary onto your property.
If you or someone you hire is doing any work within 10 feet of pepco s line which might be the case if the old cable and phone lines come in near the power line.
There will be proviso s of course but generally its to protect bt from people either over zealously digging up their cables or if suspended cable from interfering damaging with the cable while decorating or doing building work.
This is a box that was installed by the phone company when service was first installed on the house.
This video shows several things but mainly it shows how to replace an old phone wire running from the phone box service box probably located outside on th.
Find the nid network interface device.
One important safety precaution.
The nid may be located outside where the cables come into the house or may be located inside of the house in a utility area.
If you have multiple lines you will have more than one of these connectors active.
You will also need a two line telephone tester if one interface shares two lines since the second line will not be detected by a standard single line tester or a one line telephone.