I haven't used google voice, but since the article mentioned that the client was not satisfied with a google voice number I assumed they must have had some way

I haven't used google voice, but since the article mentioned that the client was not satisfied with a google voice number I assumed they must have had some way of knowing. You can take all sorts of little clues from these kinds of things. For example if a website is phrased and designed to look corporate but the only number published is a mobile and the email address ends in @hotmail.com then you know you might be dealing with a single freelancer.