It's probably not all that strange, to hear Sans' voice coming from the phone when Sans himself is standing beside them. He did once chime in on a debate about color names between Undyne and Papyrus when he was not only not in the same room, but not even in the same area of the Underground.

All those voices, all his friends...he doesn't even know where to start.

But that also means that there are no wrong answers.

"hm...let's go see how toriel's doing. i'm feeling pretty lucky today, so maybe she might even have a pie in the oven waiting for us."