If and when the company contacts you to schedule another interview, simply thank them for their time and explain that you've been offered an opportunity to attend graduate school on a full scholarship that you can't pass up. It would be basically the same thing that you'd do if you had accepted a job offer from another company between the time of your last interview and the time that they try to schedule the next interview.
If you haven't even met someone yet, I wouldn't proactively inform the employer of anything. Most people that are interviewed over the phone aren't going to be called in for a face to face interview even if the candidate thought the phone interview went well. It would likely be seen as a bit presumptuous to assume that the company was planning on calling you in for an interview before they've communicated that fact to you. Proactive communication is probably not going to save them any time-- whatever time they invested would have been invested immediately after the phone interview to decide whether to invite you for a face-to-face interview. Since there is no face-to-face interview scheduled, if you're invited and you bow out, the company will likely just schedule a different candidate in whatever slot you would have used.
From a purely selfish standpoint, it's probably slightly better for you if you want to work for this company in the future to wait and decline the interview if it is offered rather than pulling yourself out early. Best case, the interviewer likes you, asks you to interview, and then is a little disappointed that you're "the one that got away" when you decline and remembers you positively when you apply again in a few years. If you're a borderline candidate, or if they ended up phone screening a number of excellent candidates and weren't planning on interviewing you, the proactive notification makes them think "that's OK, we weren't going to interview him anyway" and that's what they remember when you interview again in a few years. Of course, in reality, it's unlikely that when you interview again in a few years anything you say or do now is really going to follow you-- you're likely going to be dealing with different people, people don't remember early stage candidates from years ago, HR files get discarded regularly, etc.