Check to see that the computer(s) you are troubleshooting are properly connected to the either a phone jack or another computer. Also make sure that if both computers are connected to phone jacks, that the phone line (and phone number) is the same for both jacks. You may want to plug a phone into each phone jack to ensure that it is working properly.

If your computers are daisy chained together, then make sure that one of them is also plugged into an active phone jack. The voltage supplied by your phoneline wiring is required for the adapters to work properly.