Cell Phone

The Cell Phone is an item in Undertale that is given to the protagonist by Toriel early in the game. Later, on the Neutral and True Pacifist Routes, Alphys mentions that the Cell Phone Toriel gave the protagonist is ancient, and promptly upgrades it. The new Cell Phone's features include a key chain, access to the dimensional boxes, texting, the underground's social network, a jetpack, a bomb defusing function, and a shooting mode.

Contacts

 * Toriel's Phone is given to the protagonist when she leaves them alone in the Ruins. Initially, the protagonist can choose from "Say Hello", "About Yourself, "Call Her Mom", and "Flirt" when calling Toriel within the Ruins. The "Puzzle Help" option will appear after wandering in the Ruins for a while. After Toriel has been asked about exiting the Ruins, she cannot be reached again through the phone until the epilogue of the True Pacifist Route where she learns about texting.
 * Papyrus's Phone or Papyrus's Hotful Helpline is obtained after dating or hanging out with Papyrus. It is mainly used to get tips or commentary, which differs from room to room. This contact becomes Papyrus and Undyne after befriending Undyne. Sans also occasionally comes into play as well. They cannot be called at the end of the Pacifist Route, presumably because their phone ran out of batteries.
 * Alphys gives the protagonist a new phone with texting if they become acquainted with her in the Lab. She gives her phone number as well, but not immediately.

Receiving Calls

 * When receiving the phone from Toriel, waiting in the room will prompt a series of calls over a period. These calls consist of Toriel getting her phone stolen by a dog, and her struggle to retrieve it. At first, she will call to say that she needs five more minutes. After waiting, she will call again to say her flour was stolen by a small white puppy, and questions what dogs would want with flour. After that, the protagonist will receive several calls indicating that the dog has also stolen Toriel's phone. Trying to call her while her phone is stolen results in the signals being deflected by a dog. Conveniently, she retrieves her phone after the protagonist leaves the room.
 * After Sans lets Papyrus know that the protagonist is a human, they can randomly receive a prank call from him. He will ask if the protagonist's refrigerator is running, and his response varies depending on what answer is chosen. This event will only happen if "Fun" values in the game fall into a specific range.
 * Alphys may randomly call to order a pizza before she is encountered. Although she is anonymous in the phone call, she can be identified by her voice. She says that she will text the protagonist the toppings but instead ends up sending ASCII art of a cat girl. The protagonist's old phone cannot receive this, so it is read out character by character. This event only happens when "Fun" values in the game fall into a specific range.
 * When approaching the area where the River Person should be at in Snowdin near the beginning of the game, the protagonist will receive a call asking for somebody whose name begins with the letter "G." The phone call then transitions into the Wrong Number Song. This phone call is presumed to be for W. D. Gaster, but it could be for Gerson, Glyde, Grillby, or Gyftrot instead. This event only happens when "Fun" values in the game fall into a specific range.
 * There is a known glitch with the Wrong Number Song where the avatar of whoever the protagonist spoke to last will show up as the ID of the call. Most players have reportedly received either Toriel, Papyrus, or Sans.
 * In Waterfall, Papyrus will randomly call the protagonist's cell phone and claim that he sequentially dialed every single digit to get their phone. He will proceed to ask what the protagonist is wearing according to a report from Undyne.
 * In Hotland and the CORE, Alphys will frequently call the protagonist to try to assist them with the fights against Mettaton and puzzles in the area.

UnderNet
UnderNet is "the Underground's number one social network", according to Alphys. She automatically signs the protagonist up for it when she upgrades their phone. There is currently no known way to access the network through the game's menu, but the protagonist can receive status updates and a friend request from Napstablook throughout Hotland and the CORE.

Alphys's handle is ALPHYS, Papyrus's handle is CoolSkeleton95, Undyne's is StrongFish91, and Napstablook's handle is NAPSTABLOOK22.

Trivia

 * If the protagonist calls Papyrus/Undyne while on a conveyor belt in Alphys's room, the text shown will read "stay in place'" as the protagonist is carried by the conveyor belt.
 * Papyrus mentions that he is a dozen followers away from double digits, implying that Papyrus either somehow has -2 followers, or has 111, which is 12 away from 99. It is either that or he may be implying that he has a long way to go before reaching it. He also mentions a troll 'besieging his online persona' with puns in a goofy font. We are led to believe this is Sans.
 * Papyrus also calls you in the middle (or late middle) of Undyne's Pacifist/Neutral battle in the entrance to Hotland, making Undyne actually stop and wait for the call to end.
 * After befriending Papyrus and Undyne, the protagonist can learn a substantial amount about their character by calling them in the different rooms. For instance, calling them in the room unlocked by completing the piano puzzle in Waterfall will reveal that Undyne made the puzzle and had been waiting for someone to complete it.
 * Once the fight with Toriel is passed, calling her will result in nobody answering. (If she is killed, you will be greeted by the answer: 'But Nobody Came') However, if she is called in the Artifact Room before the Annoying Dog absorbs the artifact, ringing can be heard in the protagonist's inventory, implying that the dog stole Toriel's phone and ate it.
 * Whenever the protagonist tries to call someone when Mettaton is present (not counting when he is out of batteries) there is an interference caused by him, but this will go away if you call someone when Mettaton is not present in the room. This same thing happens in Muffet's area.