As the interview closes, thank the interviewer for his or time. Ask what the next step in the hiring process will be.
Then ask for the interviewer's email address and send out an interview thank you email message right away, thanking the interviewer and reiterating your interest in and your qualfications for the job.
If there's anything you wished you had said during the interview, but didn't get a chance to mention, your thank you letter will give you an opportunity to bring it up.
It's important to show your appreciation for the interview regardless of whether interview was conducted in-person or on the phone.
Phone Interview Articles and Advice