The voice can be the hardest behavioral change we need to make during transition. Many of us did our part to hold up gender stereotypes and survive in the birth gender world, thus we trained our voices wrong. It takes awhile to "find" your female voice.
You know you're doing well when that's the voice that comes out when you're startled or waking from a sleep. That said, I for one always try harder when out in public or on the phone. I can get a little lazy at home.
Julie's done a remarkable job on her voice, based on where she started. I'm surprised that she thinks people on the phone would take her as anything but female. Congrats, dear!