It has become something of a cliché to describe a vocalist as ‘using their voice like an instrument’. It’s usually intended as a compliment, but it’s a pretty much empty statement, or even a self-contradictory one. After all, in music, whatever is used to produce sound is an instrument, and we don’t gain much by being told so; the suggestion is usually implicit that, even though the vocalist is ‘just’ a singer, they do some things that are characteristic of a ‘real’ musician, as though singing was somehow too easy to count.