Cell Phone

The Cell Phone is an object in Undertale that is given to the protagonist by Toriel. Later in the game, on the Neutral and 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. The protagonist can choose from "Say Hello", "About Yourself", "Call Her Mom", and "Flirt" when calling Toriel in the Ruins. After Toriel has been asked about exiting the Ruins, she cannot be called again until the end of the True Pacifist Route.
 * 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. 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 of time. 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 5 more minutes. After waiting she will call again to say her flour was stolen by a small white puppy. 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 will only happen if "Fun" values in the game fall under 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 loud character by character. This only happens when "Fun" values in the game fall under 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 only happens when "Fun" values in the game fall under 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 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, 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 it throughout Hotland and the Core.

Alphys' 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's a dozen followers away from double digits. This means that Papyrus somehow has -2 followers.