It may be similar to recent versions of iTunes where you had to first copy your tones to your iCloud Drive (in a folder called Tones or your choice), then drag them from there onto your phone in iTunes, or now to your phone in Finder. (Non-iBook Store purchased books required a similar procedure with the iBooks app.)
EDIT: I'm not sure if using iCloud Drive was actually a requirement for tones, but it was for books.