Interesting question; a few years ago, when I was beginning formal (classical?) singing training - and being thrown in the deep end with an accapella trio performing Choral arrangements of folk music, my voice was referred as "like Shirley Collins'," with the hasty added rider, "... at her best, of course!"

This, from another student of Tona de Brett's who trained Annie Lennox - who, of course, can "sing" perfectly.

Strictly speaking, I think Shirley, theoretically, can't sing in the classically trained sense of the word ... but, in practice, she has a unique "voice".