The easiest fix is to go to where the phone line enters the house
(inside), unhook everything inside, & try a phone there. This will
eliminate all the inside. If it works, try hooking up one line inside
at a time to see which one's bad. Or, just get a cheap codless phone,
if it's too hard to redo all the inside wires if that's the problem.
If, for example, you have just sprayed for insects, you may have
killed a line jack, dampness can also do this. If it still won't work
the problem is outside. The phone company will generally fix the
outside gear (theirs) for free. Hope this helps.