No, these are not really synonyms, though there may be cases where either could apply. Generally speaking: FALL OUT, implies something was inside of something else (the phone was in the bag) and it fell out of the bag. An exception would be referring to something like hair. Hair isn't inside anything else, but we can still say someone's hair falls out. COME OFF implies something was attached to something else and it became unattached.

They can’t be synonyms. The first implies that the original condition was ‘in’ whereas the second implies that it was ‘attached (to)’ or ‘stuck on’.