Close Encounters of the Third Kind

In the Sonoran Desert, French scientist Claude Lacombe (FranÃ§ois Truffaut) and his American interpreter, mapmaker David Laughlin (Bob Balaban), along with other government scientific researchers, discover Flight 19, a squadron of Grumman TBM Avengers that went missing more than 30 years earlier. The planes are intact and operational, but there is no sign of the pilots. An old man who witnessed the event claimed "the sun came out at night, and sang to him." They also find a lost cargo ship in the Gobi Desert named SS Cotopaxi. At an air traffic control center in Indianapolis, a controller listens as two airline flights narrowly avoid a mid-air collision with an apparent unidentified flying object (UFO), which neither pilot chooses to report, even when invited to do so. In Muncie, Indiana, 3-year-old Barry Guiler (Cary Guffey) is awakened in the night when his toys start operating on their own. Fascinated, he gets out of bed and discovers something or someone (off-screen) in the kitchen. He runs outside, forcing his mother, Jillian (Melinda Dillon), to chase after him.
Investigating one of a series of large-scale power outages, Indiana electrical lineman Roy Neary (Richard Dreyfuss) experiences a close encounter with a UFO, when it flies over his truck and lightly burns the side of his face with its bright lights. The UFO, joining a group of three other UFOs, is pursued by Neary and three police cars, but the spacecraft fly off into the night sky. Roy becomes fascinated by UFOs, much to the dismay of his wife, Ronnie (Teri Garr). He also becomes increasingly obsessed with subliminal, mental images of a mountain-like shape and begins to make models of it. Jillian also becomes obsessed with sketching a unique-looking mountain. Soon after, she is terrorized in her home by a UFO which descends from the clouds. The presence of the UFO energy field makes every appliance in Jillian's house malfunction and Barry is abducted by unseen beings.
Lacombe and Laughlinâalong with a group of United Nations expertsâcontinue to investigate increasing UFO activity and strange, related occurrences. Witnesses in Dharamsala, India report that the UFOs make distinctive sounds: a five-tone musical phrase in a major scale. Scientists broadcast the phrase to outer space, but are mystified by the response: a seemingly meaningless series of numbers (104 44 30 40 36 10) repeated over and over until Laughlin, with his background in cartography, recognizes it as a set of geographical coordinates (this is actually a goof; those coordinates are near Ft. Collins, Colorado...if the 40 had been a 44, the coordinates would have been plot-correct). The coordinates point to Devils Tower near Moorcroft, Wyoming. Lacombe and the U.S. military converge on Wyoming. The United States Army evacuates the area, planting false reports in the media that a train wreck has spilled a toxic nerve gas, all the while preparing a secret landing zone for the UFOs and their occupants.
Meanwhile, Roy's increasingly erratic behavior causes Ronnie to leave him, taking their three children with her. When a despairing Roy inadvertently sees a television news program about the train wreck near Devils Tower, he realizes the mental image of a mountain plaguing him is real. Jillian sees the same broadcast, and she and Roy, as well as others with similar visions and experiences, travel to the site in spite of the public warnings about nerve gas.
While most of the civilians who are drawn to the site are apprehended by the Army, Roy and Jillian persist and make it to the site just as dozens of UFOs appear in the night sky. The government specialists at the site begin to communicate with the UFOs by use of light and sound on a large electrical billboard. Following this, an enormous mother ship lands at the site, releasing animals and over a dozen long-missing adults and children, all from different past eras. Among these returned abductees include the missing pilots from Flight 19 and sailors from the Cotopaxi, all of whom have strangely not aged since their abductions. Barry is also returned and reunited with a relieved Jillian. The government officials decide to include Roy in a group of people whom they have selected to be potential visitors to the mothership, and hastily prepare him. As the aliens finally emerge from the mothership, they select only Roy to join them on their travels. As Roy enters the mothership, one of the aliens pauses for a few moments with the humans. Lacombe uses Curwen hand signs that correspond to the five-note alien tonal phrase. The alien replies with the same gestures, smiles, and returns to its ship, which ascends into the galaxy.

Question: who begin to communicate with the UFOs?
Answer: The government specialists