I know many transgender women, and I have to say, almost none of them have achieved a female voice. Without surgery it is nearly impossible to achieve if you have a deep male voice. It takes training, commitment, and years of practice. Even then, male resonance has a way of seeping through. You can't completely overcome the acoustic characteristics of the air passages from chest to nose and mouth.
Another thing I've noticed is that so many are embarrassed to even attempt to use a more female sounding voice around others. I see this same thing in foreign language classes where the majority of students are embarrassed to adopt the accent of native speakers, and, sadly, will never pass as native speakers. For anyone trying to develop a female voice, being self-conscious about using a feminine quality voice in the presence of friends and/or strangers is a sure path to failure.
It does little good to practice your female voice on a part-time basis. The reason is that you have to retrain the muscles involved in voice control to the point that they 'forget' their former habits. It's all about muscle control to elevate pitch, eliminate chest resonance, and learn feminine speech patterns and intonation. Frankly, many cannot do it, even with voice coaching. It's both a talent and a skill born of proper technique and years of practice, practice, and more practice.
Having summarized the rather depressing reality of this challenge we all face in transition, don't give up. Any honest effort you make to feminize your voice will be worth the effort, even if you can't achieve a completely passable female voice. Pass-ability depends on a collection of feminine gender cues. It is not necessary to erase every vestige of maleness from your voice, body, presentation, and mannerisms to be gendered as a woman. You only have to tip the balance in your favor.
