Truth be told... I can't pass on the phone and guess what, it doesn't matter, But I have raised the pitch and reduced resonance with practice so some cisgendered women are deeper than me and I speak more quietly. As women get older their voices deepen some because of smoking. Some cisgender girls tell me they are "Sir'd" too. When people see me they may recognize the obvious depending on their "transdar". I was a guy and now I am not. No problem. It may actually promotes understanding of transpeople.
When I go to the store, I am very well received. The other day one person gave me a big hug. WOW, I have so many friends now. I never expected this. SOooo, I guess voice is one more thing I don't need to think about. Friendly attitude and presentation are important for me.
THE ARTICLE FOLLOWS...
You can be very passable in your appearance, but if you speak in a masculine voice you've just outed yourself . . .
It amazes me how many transsexual women speak in a masculine-sounding voice, even post-ops.
They'll spend thousands on hormones, electrolysis, surgery, etc. but won't make the effort to retrain their voices. If you don't mind getting sir'd on the phone, or even in public, go ahead and talk like a man. But if you want to pass as a woman your voice is important.
Another reason to speak in a female voice is, whether we like it or not, we're all ambassadors for the trans community. People will often base their opinions of us on first impressions. If you speak in a male voice, not only will there be incongruity between your voice and appearance, but it will tend to make people relate to us more as drag queens and crossdressers—an image we need to get away from.
Any male voice can be retrained. Don't be discouraged if you're starting with a baritone! For proof that your voice can be changed, try talking in falsetto. Obviously, it sounds silly and I don't recommend talking in falsetto, but it shows even the deepest voice can be raised.
Just like your walk, you're unlearning years of doing something in a masculine way. You're retraining your throat muscles. It was two-and-half months before I started getting ma'am on the phone and it may take a year, or more, before your voice sounds good in all situations, like yelling.
Will hormones make your voice higher? Unless you started HRT at the onset of, or early, puberty before your voice changed, hormones will have no effect on pitch, though estrogen will tend to soften the voice.
What about vocal surgery? That's an option and can take the worry out of whether you'll get clocked when you speak. I know three trans women who've had vocal surgery. Two sounded good and one didn't, she sounded raspy (I guess two out of three ain't bad). As with most surgery, the outcome of voice surgery isn't certain. Explore your options and educate yourself about vocal surgery before deciding. If you don't want surgery some voice training is usually required.