If your voice is the twin of a celebrity's, but you don't benefit from the likeness (for example, not being paid for voice acting where your identities could be confused), you'd probably have a strong defense if the celebrity tries to extort royalties from you.

The rest is shades of grey. For example, if you're an incredible voice imitator and plan to do shows, negotiations would be wise. Even Weird Al asked for permission when he didn't had to, legally speaking.