Actually I think you will find that in the manual.I'm pretty sure it is recommended that each station has at least 2 trunk keys so they can be on a call and have a second come to the set.I had a customer that had the same kind of problem.They had direct trunk appearances for 6 lines but the other 6 lines were utilized with 1 trunk key.If they were on a call on the trunk key and a second call came in that wasn't a direct trunk key the calls would kick back to the auto attendant because there was nowhere for it to go.