You will have to isolate the phone line when you test it. All of the
phone jacks in your house are typically in parallel. Which means if you
have a problem on one line, all of the others will be affected.
Similiar to if you had a short on one of your electical outlets, the
breaker will trip and affect all of the outlets on that circuit until
you isolate the problem.
Check to see if all of the phone lines in your house run to the
interface box. If it does, you can isolate the wires and try each one
until you find the problem.
If your line runs from jack to jack, you have to go to the first jack,
remove the wiring to the rest of the house, and test the line there,
then repeat at the next jack until you find the bad line.