"No Line" on the phone's display means that the telephone is not sensing the approximately 50 VDC on-hook power from the OBi. It can be a bad telephone, a loose phone plug, or a bad cable (run over by a chair, chewed by a pet, etc.). Those are the first things to check. If you have another telephone, swap out both the telephone and the modular phone cord between the phone and the OBi, and see if the trouble goes away. If it goes away, then, using the replacement phone cord, unplug the test phone, and plug in your original phone.
If the problem comes back, it's a bad phone. If it works now, it was either a bad cord or a loose connection. If neither phone works, then reply with that news.