Most lines especially in the South had been destroyed by both sides to prevent their use by the other. That is on top of the fact there were few lines running North to South to begin with.
Now once the information made it to the extensive Military only telegraph system put in place by the North, it was a different story.
The Capital of the Confederacy was Richmond, at the most a day's ride to the Union lines.
[Regarding a free black woman posing as a slave in the Davis household.] Information about her is scanty. One good source is Thomas McNiven, who posed as a baker while making daily rounds as a Van Lew agent in Richmond. From him, down the years, came the report that she “had a photographic mind” and “Everything she saw on the Rebel President’s Desk, she could repeat word for word.”
Jefferson Davis’ widow, Varina, responding to an inquiry in 1905, denied that the Richmond White House had harbored a spy. “I had no ‘educated negro’ in my household,” she wrote. She did not mention that her coachman, William A. Jackson, had crossed into Union lines, bringing with him military conversations that he had overheard. In a letter from Major General Irvin McDowell to Secretary of War Edwin M. Stanton, “Jeff Davis’ coachman” is cited as the source of information about Confederate deployments. A butler who served Jefferson Davis also made his way to Union lines.
Although McDowell and other Union generals could attest to the value of the Black Dispatches, the best endorsement came from General Robert E. Lee. “The chief source of information to the enemy,” he wrote, “is through our negroes.”
Sounds like these "docile, intelligent, and civilized agricultural laborers" made a pretty useful contribution to the war effort. At least three in Davis's household alone risked their lives to bring down his rebellion and its "peculiar institution."