Rex, just like with GG (genetic girls) WALDT. I am with you on the voice thing, however
I've seen a few TS providers and most of them make an effort to sound and appear feminine. If they are young and/or started on hormones when they were young, they are more likely to have the really soft feminine voice.
The easiest route is to simply call them, and then you'll hear their voice. Ask a few questions about scheduling or location. If you don't like the voice, you can simply hang up (kind of rude) or you can say "I'm sorry, this is my first time and I can't go through with it."
Also note that the more feminine in voice and appearance, the more you can expect to pay. And that doesn't necessarily mean you'll get a fantastic session either. There are a couple TS providers who visit Dallas frequently, very feminine looking, and charge 300-400/hr. But they have lousy reputations for service.
If you want to research more TS reviews, check out mine and Tom Clubs. We probably have the most reviews here in the Dallas area. Good luck!