I watched in System Status while making test calls. It seems to randomly happen, but always picks up the next line in sequence... (1st test call, card 1, port 9 | 2nd test call, card 1, port 10,,,,,). All lines have dial tone and good battery voltage from the CO. I have not set the unused ports out of service. I have never done that and not had an issue, but it is worth trying. I'll give it a shot.