Some days later, in the evening, Frisk was going back home a familiar female voice stopped him. The woman was pointing at Frisk with gun and commanding him to slowly turn around and come with her. He did turn, but didn't look like he wants to come with her. Out of their short conversation, the only witness of which was invisible for anyone Gaster, you could conclude that the woman was Frisk's mother, at least biological, but young man was more than cold for his parent neither it seemed she much cared about her child.
Suddenly Frisk's phone called. It was Sans. Woman allowed Frisk to answer it but warned him to watch his words. She knew the monster can teleport. In their conversation young man indeed didn't say anything what would tell about situation. Or at least you could think so... they had a weird pun line which meant that one or another is in danger.

Just some moments later after Frisk ended their conversation, Sans teleported to him with Papyrus which made woman to shoot out of surprise.

Woman won some time with her shield and run to the camion which was waiting for her near by. They couldn't stop her or follow her... but now was the time for answers.
They came to Grillby's, as it was the safest place for the dialogue. And Frisk had no choice but to give what monsters asked for.