The phone line rather like water pipes become your responsibility as soon as they cross the boundary onto your property.
Telephone line coming out of the floor.
You will need to test them all to figure out which is the defective one.
Find the nid network interface device.
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.
This is a box that was installed by the phone company when service was first installed on the house.
I really doubt there is an electrical box behind the drywall.
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.
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.
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.