Not an easy promise to make, when you have no idea who this man is or even particularly where you are right now. So Frisk doesn't make it, even though they're obviously considering the idea. This guy needs someone to help him out. It doesn't sound like there's anybody else - they'll ask Mettaton to come, maybe. It won't be that big a deal.

Frisk. They hold the phone up for inspection, then point to themself.