Most inventions are not something brand new, but rather a new or improved use or mfr of existing materials or products. The telegraph used electricity which was known and distant communication which was known. Who invented smoke or flag signals?
I don't know the cut off, but at some point an improvement to an existing product ("an implementation") becomes a new patentable invention. For example, the early IBM punched card contained 45 columns of round holes. They improved the design to be 80 columns of rectangular holes on the same size card, this got them a patent.
The transmission of telegraph signals over phone lines wasn't new, but they were sent over a special reserved segment of the lines. The use of the voice line to send signals, without any special modification of the line, allowed for much higher speed. To me, the development of the "modem" and transmission code and protocol was a significant improvement and did constitute an invention. IBM announced this as a product in 1954. I don't know what AT&T had, if anything, at that time.
Note that in 1954 the primary use of such data transmission was not for computers (too few of them at that point), but rather card tabulating machines.
Regarding acoustic couplers on modems, they existed prior to connect other devices to the telephone network. Of course, one could lease Bell provided Dataphones for your own terminals, or a Bell provided terminal. Teletype began to make CRTs in its last years.