Cell Phone

The Cell Phone is an item in Undertale that Toriel gives to the protagonist 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 upgraded 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 appears 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 upgrades the protaganist's 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 prompts 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 calls to say that she needs five more minutes. After waiting, she calls again to say a small white puppy stole her flour, and questions what dogs would want with flour. After that, the protagonist receives 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 asks if the protagonist's refrigerator is running, and his response varies depending on what answer is chosen. This event only happens 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 texts 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.
 * In Room 70 (room_tundra_dock), the northernmost part of Snowdin, the protagonist's Cell Phone may randomly receive a call. The phone call begins asking for somebody whose name starts 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 shows up as the ID of the call. Most players have reportedly received either Toriel, Papyrus, or Sans.
 * In Waterfall after solving and crossing the second bridge seed puzzle, Papyrus calls the protagonist's cell phone and claim that he sequentially dialed every single digit to get their phone. He proceeds to ask what the protagonist is wearing according to a report from Undyne.
 * In Hotland and the CORE, Alphys frequently calls the protagonist to try to assist them with the fights against Mettaton and puzzles in the area.
 * In the Elevator to exit the True Lab, an "unknown voice" calls the protagonist, saying: "&lt;Name>, Are you there? It's been a long time, hasn't it...? But you've done well. Thanks to you, everything has fallen into place. &lt;Name>... See you soon."

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 reads "stay in place" as the conveyor belt carries the protagonist. This was fixed in v1.001.
 * Papyrus mentions that he is a dozen followers away from double digits, implying that Papyrus either somehow has -2 followers, or has 111. It is that he has a lot of followers, or he may be implying that he has a long way to go before reaching it. It's possible he just has a bad concept of numbers. Papyrus mentions that a troll is "besieging his online persona" with puns in a goofy font. This is most likely Sans.
 * If befriended, Papyrus also calls the protagonist 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 characters by calling them in the different rooms. For instance, calling them in the room unlocked by completing the piano puzzle in Waterfall reveals that Undyne made the puzzle and had been waiting for someone to complete it.
 * If a call is made when a Gaster Follower is in the room, the phone does not turn on.
 * Once the battle with Toriel is completed, she can not be reached even if she is still alive. 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 interference caused by him. This same thing happens in Muffet's area.
 * There is a variable in Undertale that turns off Alphys's notifications. This is used in the Genocide Route, as one does not befriend Alphys. Toby Fox said he intended to make the player able to toggle this ability in-game, but he didn't implement such feature.