Here's how I see it:
A singer that cannot play a musical instrument is a singer and not a musician. A person that can both play an instrument and sing is both a singer and a musician and if they can write music, they are a composer as well. The human voice is instrumental in singing, but it is not an instrument.
A singer that cannot play a musical instrument is a singer and not a musician. A person that can both play an instrument and sing is both a singer and a musician and if they can write music, they are a composer as well. The human voice is instrumental in singing, but it is not an instrument.