Prior to the use of communication satellites, we had only two systems to defeat the curve of the earth. We had terrestrial wired systems, and we had microwave relay systems with towers on hilltops about 30 miles apart. But these systems were not great. Spanning just the North American continent took 107 hops by microwave relay. Prior to 1954 transatlantic cables were mostly fiscal and/or technological failures. Until the 1950s transatlantic telephone service was primarily radio-based. Technology had to catch up to the idea, but in the years after Clarke's proposal, the world began considering another approach. (Which we ironically reversed over the last decade)
Sputnik I was launched by the USSR in October of 1957, and the USA launched Explorer I in 1958. These were satellites, but not communication satellites. In December of 1958 we launched Score under the Defense Department's ARPA program (Advanced Research Projects Agency). It was a shiny metallic mylar balloon 100 feet in diameter. It was in the spirit of, if not the design of some passive reflector design proposals written by J.R. Pierce around 1954. The actual unit was designed by Kenneth Masterman-Smith. It contained two tape recorders (for redundancy) and four antennas two for transmission and two for reception. The frequencies used were 150 MHz for the uplink and 132 MHz for the downlink. Its batteries lasted 12 days and it then burned up on re-entry in January of 1959. It broadcast the first human voice communication from space on December 19th. President Eisenhowers words were as follows:
"This is the President of the United States speaking. Through the marvels of scientific advance, my voice is coming to you via a satellite circling in outer space. My message is a simple one: Through this unique means I convey to you and all mankind, America's wish for peace on Earth and goodwill toward men everywhere."
That's what he said, but what he meant was the U.S. now had the ability to deliver a nuclear weapon from space. The Russians took this poorly. More here. AARPA moved on with a project called Advent and Echo to mixed success. Echo was another 100ft passive reflector mylar balloon this one launched in August of 1960. It remained in orbit until May of 1968. Echo II was a bit more interesting. It was bigger at 135 feet in diameter, and was launched January 25, 1964. It came down in 1969. Following Echo, passive communications satellites were abandoned. Telstar 1, the first production active communication satellite was launched on July 10, 1962.