If you are using a a Phone device (not an ATA) this generally indicates that there is a network connection issue. The indication generally means the cable is unplugged somewhere, or that it is defective.
Many times it is a matter of unplugging the cable from the back of the phone, and plugging it back in, and insuring that it has clicked into place. If this doesn’t fix things, then repeat the process on the other end of the cable. Occasionally there are other links on the connection, these should also be checked all the way back to wherever it is connected.