With the box blade and one tooth down, you will have the weight of the box blade. Don't forget that. I used to bury phone lines too, all day, six days a week. We didn't work Sunday because it inconvenienced people at their homes. But, if we had a mile long run across a farm, we'd be there doing it. I think the box blade apparatus will do the job, with a long tooth. They make a phone line, five pair that is shielded, has a metal wrap around the outside for direct burial. My brother is a phone man. If you catch a phone man you can ask him for a piece. They keep leftover pieces off large spools, rolled up at the phone office. They know how long they are. 5 pair shielded in conduit would be bullet proof, I dare say