Back in the day, it was common for households to have multiple landline phones or handsets corresponding to the same number or phone line, allowing for easy conference calls. (Or, being able to pick up the phone in the bedroom instead of having to run all the way to the living room or kitchen and hope you got there in time.) This led to obvious opportunities for eavesdropping. If you want to eavesdrop on another person's phone call, just hide in another room that has a phone extension, quietly pick up, and hold your hand over the receiver while listening in.
For added drama, old landline phones made it possible for the eavesdropees to realize they were being eavesdropped. In addition to the slight click a phone could make when it connected to the line, older phones tended not to have great microphones and could pick up a wide variety of environmental sounds. And, finally, the eavesdropper might lose their cool and shout something at whoever they're listening to.
Common offenders include spouses eavesdropping on their unfaithful spouses' Intimate Telecommunications, nosy children spying on their parents or siblings, and Overprotective Dads spying on their rebellious teen's plans. This isn't limited to members of the household, however. It may also be used by an investigator or stalker staking out a target by hiding in their home while their target makes a phone call.
This was most common in the 1980s and 1990s, when having multiple phones in a house become more widespread. These days it's a Discredited Trope due to the obsolescence of landlines, but was once Truth in Television.
An older equivalent can be found in party-line phones. Party-lines were multiple households or apartments who all shared a single phone line. They were cheaper than a single house line, and were common from the early days of telephones all the way into the 1970s. The trope of a gossipy neighbor eavesdropping on a party line call was especially common in the 1950s and 1960s, but is pretty much a Forgotten Trope today, since most landlines are single party, and that's if a house even has a landline.
- Foxtrot: Jason was fond of eavesdropping on his teenage siblings' phone conversations. In one strip Pete and his girlfriend are inexplicably speaking in obvious code phrases before cutting to Jason and his friend listening in with the comment, "They're on to us." In another, Paige is trying to persuade their mom to get a second phone line:
Paige: "A second line wouldn't be..." [shouts into phone] "TAPPED!"Jason: [ears smoking] "Medic..."
- The Incredibles: Helen picks up the landline to overhear a secret conversation between Bob and Mirage, and suspects that Bob has been having an affair with Mirage instead of working at his old job, despite the two actually arranging the details for his next mission.
- Ferris Bueller's Day Off: When Ferris attempts to bluff his way into a fancy restaurant by using someone else's reservation, the waiter attempts to listen in on Ferris's phone call to verify what he's been told by Ferris and his girlfriend. The person he's on the phone with is Cameron pretending to be the police.
- It's a Wonderful Life: George comes to visit a young Mary while her suitor is on the phone from New York. Mary's mother, who's keen to set her up with the suitor, listens in on an upstairs line and grows increasingly concerned when the conversation doesn't go well and Mary makes George talk to him instead.
- In one scene in Mean Girls, all four of the titular mean girls are on various landline calls with each other. Cady invokes this by calling Regina and secretly having Gretchen listen in on another line to find out what Regina's been saying about her behind her back. It's nothing good. Since Regina also insulted Gretchen's best friend Karen, she calls her to let her know... and then Regina calls her, too, which leads to this trope happening again by accident.
Karen: It's Regina. She wants to go out but she told me not to tell you.Gretchen: Do not hang out with her!Karen: Why?Gretchen: You don't want me to tell you.Karen: You can tell me! Hold on. [she tries to click the button to switch lines] Oh my God, she's so annoying.Gretchen: Who is?Karen: Who's this?Gretchen: Gretchen.Karen: ...Right, hold on. [she clicks the button again, this time successfully switching back to Regina's line] Oh my God, she's so annoying.Regina: I know, just get rid of her.
- Real Genius: When Mitch calls home after being humiliated in front of Prof. Hathaway by Kent, Kent and his two friends pick up an adjacent phone to spy on him as well as recording it for later humiliation.
- The Return of the Pink Panther: Inspector Clouseau picks up a phone in an attempt to spy on the wife of Cat Burglar Charles Lytton, subverted when we learn the conversation is occurring inside the house to draw him to a false lead.
- In Snow Dogs, Rupert tries to eavesdrop on a call between Ted and his mother, but they both twig on immediately.
Ted: Rupert, get off the phone!Rupert: I'm not on the phone! (winces)
- In Splash, Allen gets a call from his girlfriend Victoria. As we hear from what Allen says, she's moving out and hasn't mentioned it until now. Allen's brother Freddie listens in, even though they're in the same room, and is just as surprised when he finds out about the news.
- In Submarine, Oliver is convinced his mother is cheating on his father, so he hides and listens in on the second phone line to all his mothers' phone calls. He discovers that she has some kind of relationship with Graham, and starts stalking them based on the information he retrieves from these calls.
- Trading Places: Billy Valentine listens in on the Duke brothers' call to their minion, Clarence Beaks, and finds out where Beaks will be after stealing a copy of the annual crop report.
- White Christmas: The housekeeper at the inn eavesdrops on Bob Wallace's phone call with a show host, but doesn't hear the part where he refuses to make a publicity stunt out of helping the retired General.
- Youth in Revolt: Nick calls up Sheeni to tell her excitedly that he's on the run from the law for arson and is moving up to Ukiah to be with her; unfortunately her conservative religious parents are listening in on the other line and immediately ban them from seeing each other and send Sheeni off to boarding school to boot.
- In Albertine and the House of a Thousand Wonders by Frank Reifenberg and Jan Strathman, the headmistress of an Orphanage of Fear uses her handset to eavesdrop to a call allegedly from a trustee to two boys from the orphanage. As it turns out, the "trustee" is Albertine, a girl who escaped just several days earlier, and she is calling her best friends to tell them how to get to her hiding place. The headmistress locks the boys up and goes to bring Albertine back to the orphanage instead.
- A party-line phone example in Anne's House of Dreams. Anne notes that telephones have finally reached Avonlea (it's around 1890, telephones have only been around in Canada for about ten years at this point). Of course all of the busybodies listen in on calls. Anne recognizes the sound of a clock in the background during a call, a clock that she knows belongs to a busybody that's listening in. She asks if the person she's calling has a new clock, cue the click of the busybody hanging up.
- One Dave Barry column had this situation, though it was less eavesdropping as it was everybody on the party line talking at the same time and responding to conversations not meant for them.
- Clue: In book #5, chapter 2 (Midnight Phone Calls), Miss Scarlet slips away to make a private phone call to someone who'll make a secret late-night delivery to the mansion for her. One by one, the other guests also each pick up a phone somewhere in the mansion and listen in on it, with only the first two to do so hearing where the deliveryman will be hiding what Miss Scarlet had ordered. Her favorite brand of jelly beans, as it turned out, though the way she voiced it, it sounded like she was talking about real jewels.
- In Day Of The Jackal, the Jackal has taken shelter in the mansion of a woman he seduced. When he makes a call to his informant, he always makes sure the woman falls asleep before making a call from a phone in the other room. During one call, he hears a click on the phone, and realizes the woman has been listening on another phone. When he returns to his room, he finds one of his suitcases opened and parts of his Scaramanga Special rifle on the floor. He immediately kills the woman, packs his cases, and flees, but killing her ultimately ended up harming him; before, when he entered France, he was attempting to assassinate Charles de Gaulle and the search for him was carried out in almost complete secrecy. After this murder, however, it became an open, public manhunt.
- In The Milagro Beanfield War everybody in the town of Milagro, New Mexico, is on a party-line (successive mayors get elected on the campaign promise 'No More Party Lines'). The radical faction is alerted to the presence of a state agent going to a meeting of the conservative faction by overhearing it on the telephones.
- In Doctor Who Meets Scratchman, the Doctor calls his companions from a payphone outside a post office. Because of the way the system worked in the seventies, the suspicious postmistress is able to listen in on the call. Which is exactly what the Doctor wanted.
- Arrested Development: In the Season 1 episode Marta Complex, Lucille eavesdrops on Buster's phone call with Michael, but her cover is blown when Michael hears her maid Lupe asking her a question.
- Friends: Rachel was able to learn about Chandler and Monica's secret relationship by picking up the landline and accidentally overhearing the two flirting with each other.
- Sophia listening to her roommates' conversations through the extension in her bedroom was a Running Gag in The Golden Girls.
- How I Met Your Mother had an episode where Marshall's mother and older brother were using phones to listen in on the conversations Marshall was having with the rest of the gang about the recent developments between Ted and Zoey.
- In an episode of Malcolm in the Middle, Lois wants to know who burned her new dress and subjects Malcolm, Reese, and Dewey to a series of interrogation techniques. Malcolm calls Francis for support in how to resist her techniques, until he hears Lois' breathing over the phone...
- Throughout his tenure on M*A*S*H, Radar performed this trope. Oftentime he did so to simply be quick to pick up on orders, but about equally as often he really WAS snooping.
- Stranger Things: While Mike is talking to El on the phone in the basement his mother listens in using the phone upstairs. When he falsely claims his grandmother is sick rather than explain the real reason he's avoiding El for the day, his mother gives herself away by worriedly asking if something is wrong with Nana.
- The Amazing World of Gumball episode "The Spoiler" has Gumball and Penny talking over the phone, only to discover all of Gumball's family and most of Penny's were listening in through the other phones—as was one wiretapper, apparently a government agent, that everyone ignores.
- American Dad!: In "Roger and Me", Francine (suspicious about what happened during a trip to Atlantic City) uses this technique to listen in when Roger calls Stan at work.
- Courage the Cowardly Dog: Courage uses the phone to listen in on Muriel talking to a suspicious mattress salesman.
- In an episode of Family Guy, Brian talks on the phone with his girlfriend Jillian. After she says something particularly stupid, he hears someone snickering.
Brian: Stewie, are you on the line?Stewie: Yes.Brian: Jillian, I'll talk to you later.
- The Powerpuff Girls had the mayor listening in on the Professor and Ms. Keen flirting with each other for hours because he couldn't bring himself to interrupt despite the fact that he needed the girls to save the day.