Even-more direct follow-up hint (with one number worked out):
If the first three numbers are all 1 (as low as they can be) and the fifth question is false, then their sum is 3 and the fourth number is either 1 or 2. Because that doesn't isolate a single number, that can't be right - so the fourth number must be 9, with the fifth question being true and the sum of the first three numbers being 8.
From here, the rest of the solving should be a lot easier - there's only so many combinations of the numbers on this phone that can total 8!