No dial tone

We are not getting any dial tone on our phones. Voicemails are coming through and we get alerts on the TV that someone is calling, but the phones sound dead when I pick them up. I have disconnected all the phone and tried a corded phone. Attaching it to the phone jacks in the walls does nothing. If I plug the corded phone directly into the modem, I can get a dial tone. How do I fix this??