In this CNN article there is mention of a utility or app called "PhoneClean" for iPhones (see suggestion #5). Is there such a thing and would it be worth trying? No such app shows up in the App Store that I've found.
5 ways to expand your iPhone's storage beyond 16 GB - Feb. 17, 2016
5 ways to expand your iPhone's storage beyond 16 GB - Feb. 17, 2016