Most definitely the voice, and also the hair and body shape (tall, thin, etc.).
I have no trouble recognizing my family, friends, and about ten colleagues (except that one girl who changes her hair everyday, I can never recognize her).
With clients it's different. They have to come everyday for me to recognize them, and even so, it's because i'm at work and in that environment chances are the person who comes in is who i think they are.
Recently my husband & I were walking in the town center and i saw a man that looked like a customer of mine, but i wasn't sure. I tried to look carefully but you can't stare too much or the guy will think you're nuts or something
So i said hello, just in case, but the man barely replied so that might have been somebody else.
My husband asked me "do you know him?",
"well, maybe" 