Voice training is the process of training your voice to sound a certain way. Here, this pertains to making your voice sound more masculine or feminine. While voice training is more common among trans-feminine individuals (as estrogen does not change your voice), some trans-masculine individuals who opt not to go on testosterone (or are otherwise unsatisfied by the voice drop typically provided by going on testosterone) will train their voices to sound more masculine.

Google Docs - Transmasc Voice Rundown Scinguistics
Speech Voice Lab - Transfeminine Speakers
Speech Voice Lab - Transmasculine Speakers
Trans Guys - TESTOSTERONE AND THE TRANS MALE SINGING VOICE
YouTube Channel: TransVoiceLessons
YouTube Video: How To Achieve A Natural Male Voice
YouTube Video: Male To Female - Voice Feminization