Well, before I trained my voice to be deeper and adopted the mechanic's accent, I passed as a girl often without trying, and almost every time on the phone. Actually had a coworker who wanted me to be less feminine coach me with having a deeper voice.
The way I understand it, cadence matters most. And you can get that by trying to work with a sing-song voice, starting with songs sang by women. If that's the One True Trick, then I'd suggest some Madonna for you.If you're in the tenor range, achieving an alto isn't that difficult, but it has to come naturally. If you're a bass, then you're better off learning how to talk like a sick person when you're dressed. I had the opportunity to audition for a band whose guitar/singer is a trans woman, but the rest of the band isn't. She wanted a trans bass player, though. So I went and listened to their music, and the singing voice was so obviously a male voice overreaching to try to sing like a woman that I was like "I'm NOT going to be in this band". And every song was about being trans... Don't get me wrong, I want a trans band, but I want it to be a cover band, and when original songs are performed, they should be regular songs. Regular songs change meaning when performed by a trans group.
I'm trying to unlearn the trained male voice I talk with, and it's been difficult. For one thing, whenever I try to let it go, I get flashbacks to the coworker telling me I shouldn't be so feminine and feeling hurt by it. I've had an easier time dropping the swagger that was also trained.
