The HTC patent consists of a protocol where the OS would keep track of who you call regularly without voice tags, and if you exceed a certain number, the dialler will automatically prompt you to record the voice tag.
For the legalese version, see below:
A method for setting a voice tag is provided, which comprises the following steps. First, counting a number of phone calls performed between a user and a contact person. If the number of phone calls exceeds a predetermined times or the user fails to perform a voice dialling before calling to the contact person within a predetermined duration, the user is inquired whether or not to set a voice tag corresponding to the contact person after the phone call is complete. If the user decides to set the voice tag, a voice training procedure is executed for setting the voice tag corresponding to the contact person.
Read the full patent at the USPTO here.