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.
The phone cable is very low on the wall only about 1 1 1 2 inches from the floor and the wires come straight out of a small hole only big enough for the wires.
Find the nid network interface device.
If you have multiple lines you will have more than one of these connectors active.
You will need to test them all to figure out which is the defective one.
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.
This is a box that was installed by the phone company when service was first installed on the house.
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.
Outdoor nids are typically located near your power meter or at the location that cables from the street come into your house.
One important safety precaution.