It may have been the telegraph. regardless, theres a whole documentary on it that discusses the huge research that went into cables ...
You seem to be very confused about transatlantic cables. Perhaps you should watch your documentary again.
... when the first cable was laid under the Atlantic and people tried to make a transcontinental telephone call, the sound came out skewed in time, noisy and garbled. They checked the cable along the ocean floor and it was in perfect condition and insulated.
That’s not even remotely true. Why are you trying to revise history?
Here’s the Wikipedia entry on transatlantic cables. It looks historically accurate to me, and note it references reliable sources.