After the whole encounter of Flowey and the froggit, Toriel had been leading you through the ruins, since she knows all the puzzles in here. Anyway, she had made you walk through a hallway alone. And you've successfully made it. However, she didn't leave you, she hid behind a pillar. And when you arrived at the end, she showed up.

"Greetings, my child. I didn't leave you alone in the hallway. I have some important business to attend to right now, which is why I'm gonna give you a cellphone. But you should wait here until I'm back."

Toriel gives you a cellphone

"Be good, alright?"

She pats your head as she walks away with a warm smile yet with a hint of worry