As a temp solution, in eManager, go to Advanced Configuration > Trunk > Assignments and on the Basic tab of each CO line, #12 Hunt Order, you can change that so that line 5 is number 1, line 4 is 2, etc. and line 6 should be 6 so that its the last line to be picked. Keep in mind that it still is possible that line 6 can be used for a voice call.