I recieved my google voice number about two weeks ago (after many, many months of waiting). To get a google voice number you still need to request and invite, and google is currently stating that there is a short wait to get your google voice number.
Brief Intro to Google Voice: Here is the video that briefly explains the concept of google voice.
Now the good stuff...Classroom Applications:
Oral Homework Response - Teachers could either hand out a question(s) for homework, and instruct students to call the teacher's google voice number to leave an oral response to the question. Particular parameters can be set by the teacher regarding length and expectation.
Oral Response for Math - Students can explain how they were able to solve a particular math problem and walk through the steps while explaining their thinking process along the way.
Student Messages Turned into Text - a google voice account can be set to turn voice messages into text. Students can leave oral responses to questions and the teacher can read through the responses and bring in the best work to share & model to the class the next day or prior to an exam.
Student Call In - Teachers can customize the greeting to their google voice number, and if they know the phone number their students are calling from, they can leave a customized message for just their students. Students can then call into the teacher's google voice number to get directions for an assignment, updates for class, tips for an upcoming test, or a quick oral review from the teacher.