One thing leads to another, and eventually Mettaton mentions to the voided one that somehow, two young skeletons seem to have appeared out of thin air. (Not to mention Mettaton himself remembers the Castle and spending so long with Gaster.)

He does debate with himself for a long, long time whether to speak up at all, but time as he perceives it and time in reality don't sync up at all. They could use someone who was there also to speak with.

And he could use someone to talk to, period.

He only has one means of communication available to him. And he knows Frisk's number from Mettaton, who can't keep his mouth shut for anything.

Hello, Frisk.