Crossword puzzle clue "use a line on"

Add vertical line 13 (word 3) and vertical line 3 (word 1) together and enter the answer to the resulting clue on this line

Add vertical line 9 (word 2) and vertical line 7 (word 2) and enter the answer to the resulting clue on this line

Add vertical line 1 (word 1) and vertical line 15 (word 3) and enter the answer to the resulting clue on this line

Add vertical line 15 (word 1) and vertical line 1 (word 3) and enter the answer to the resulting clue on this line

Last under use or erode under use

In use, as a phone line

Use a line on

Reason to use a hot line