What does "out of the box" mean?

As it is sold new, without additional extras.

That is the meaning in the context that you gave, but be cautious. That is not the only meaning of the idiom.

In this case it means that the $800 is for the phone itself. It does not include a cell phone data plan which must be purchased separately. So there will be extra charges in addition to the $800 already spent.

Brand new. Never used. NIB: New In Box.

Don’t gloss past the prudent caution dispensed by @marinelife. While “out of the box” usually refers to something ready to function or operate the moment it is unpacked, there is also anoter use for the phrase along the lines of “Both Obama andTrumps’ ideas are ‘out of the box’.”

Funny, I would’ve taken it to mean that it is used and not new. Especially since I have always seen, as @kritiper mentioned, NIB for New in Box. So I would’ve taken it to mean that it is use. Who knows, it may also be out of box because it was stolen and it’s their way of saying that they don’t have the details that came with that particular item, or proof that they actually purchased it. Beware.

Out of the box usually mean something especially any electronic goods or software which is ready for immediate use after recent purchase. However the idiom ‘think outside the box’ implies imaginative and creative ideas, which are not conventional.