Charles Hill, the Postmaster General, spoke on the opening circuit, which is the ceremonial end of it. The other end was 51 repeaters potted in steel on the seabed between Oban and Clarenville, valve amplifiers expected to run twenty years with nobody coming to look at them. 588 calls went through on that first day in 1956, over 35 speech circuits plus one carrying telegraph.
The ship end of it deserves a line: HMTS Monarch, about 8,000 tons, paying out at a few knots and slower when the North Atlantic wanted a say. Cable ships work to weather, not timetables, which as a former signalman I find hard to forgive and completely understandable.
Okay, the speed of it: about 30 milliseconds each way down that cable, a third of a blink. Bounce the same call off a geostationary satellite later and you're up 36,000 km and back, roughly a quarter second each way, which is exactly why people on those circuits kept talking over each other.