I think common reasons to fail the telephone interview are for theory fails, LST fails, having already failed the assessment before and perhaps if you are deemed to have insufficient language skills. I am unsure about the last one as either the language test/sentence thing is new or is not done with native English speakers (or it may be down to the judgement of the interviewer).
FWIW, 'interview' is the wrong term for the telephone call. It really is just a quick fact gathering exercise.