One question what will happen for pawn promotion
Mine is back in its box at the moment (to make room on our dining table) so I searched the Phantom Discord channel and found:
Simply place your pawn on the opposite location, and the app will display the piece you wish to retrieve. If the piece is available off the board, it will automatically replace the piece. However, if it is not available, you will need to manually replace your pawn with the queen.
They are also prototyping a feature where you leave the spare Qs on graveyard squares, so they promotion can be automatic. I'm not sure about under-promotion, although this is quite rare (see https://www.facebook.com/billygatechess/posts/pfbid0zbL3sqWCEpL6kqs6GBP1ke5T4RwpZ96ULinjES79KcZzPCfe6DxwkfrgNpGjBNsl).
One question what will happen for pawn promotion