Originally Posted by KoryAK
Your cake is settling extra because without internal support (JUST dowels does nothing for gravity) and now the fondant piece is too large. To save this cake, you have to disassemble, reassemble with a board in the middle as the pps have described, and redo it. Sucks, but that's the truth.
Here's an excellent video about cake construction:
For tall cakes like your phone box, you must have a cardboard for every 4" of height. You can carve the cardboard to the same size as the cake as you are stacking. When you cut the cake, you simple cut from the top down until you reach your top-most cardboard. Cut and serve the slices. Remove the cardboard. Continue on down to the bottom.