is there any chance you have DSL?

More than 3 filters? Installed wrong?

Check for phone line sounds when you have the DSL modem on and off. If you hear sort of a static on the phone that goes away when the modem is turned off think of a DSL filter/splitter issue.

If you have more than one device in an area, us only one filter there.

You really need to plug a known good phone and cord into the NID outside the house. If you don't have one, ask for one to be installed.

The protector should essentially be open on both sides to ground.

The "protector" which is on the telco side can also cause issues. You might also have one inside if the wiring is old. It was the black thing where the phone terminated inside the house. It has solid carbon under the screws.

The real important test that's hard to do is effectively the resistance looking into your house with the NID disconnected. generally, it's easy to do some of the disconnections with the NID itself, but a blank cord will do what you have to do.

If you have a few phones, try to measure the resistance looking only into the house with the phones disconnected. That measurement should essentially be an open circuit. This is where you need to start looking for cob webs in the sockets. You can do this test with the phone connected but off hook.. I forget the magic mumbers your looking for. I think it's a few megohms max.

If overhead lines, then spiders in the box there too.