Find out the person who called you. Then call them back. Tell them thank you for their calling, how interested you are by the position, also tell them you are the very person for this position. Try to get to know when the interview will be held, who is the interviewer, may I get to know his telephone number or email address. Try you best to get more information. Keep on contacting them. Call them again if you don't receive any call 1 day after the date he told you would have the interview.