Cell Phone

The Cell Phone is an object in Undertale that is given to you by Toriel. Later in the game, if you're in a non-Genocide run, Alphys mentions that the Cell Phone that Toriel starts you off with is ancient and will promptly upgrade your phone to have access to the dimensional boxes, a key chain, texting, and the underground's social network.

Contacts

 * Toriel's Phone is given to you when you she leaves you alone in the Ruins. You are given the conversation topics of "Say Hello", "About Yourself", "Call Her Mom", and "Flirt" at the beginning of the game. Once you've fought her in The Ruins Exit, she can not be called again until you've reached the end of the Pacifist Route.
 * Papyrus's Phone or Papyrus's Hotful Helpline is given to you after the date with Papyrus, it is mainly used to get somewhat useful knowledge or tips which differs from room to room. This contact becomes Papyrus and Undyne after you become best friends with Undyne in a non-Genocide run, making it so you can also talk to Undyne. At the end of a Pacifist Route, you can no longer call them since their phones presumably ran out of batteries.

Receiving Calls

 * After Sans lets Papyrus know that you're a human, you can randomly receive a prank call from him.
 * When you approach the area where the River Person should be at in Snowdin near the beginning of the game, you receive a call asking for somebody whose name begins with the letter "G". This is presumed to be W. D. Gaster but it could be Gerson. The phone call then transitions into the Wrong Number Song.
 * In Hotland and the Core during a non-Genocide run, Alphys will constantly call you to try to assist you with the fight against Mettaton and puzzles in the area.

Social Network
You are automatically signed up to the Underground's number one social network after Alphys upgrades your cell phone to include it. There is currently no known way to access the network through the game's menu, but you constantly receive status updates and a friend request from it throughout Hotland and the Core.

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