A phone job interview is a prescreen a hiring professional conducts, to determine if they will process your application to next step of the interview process. Meaning, they will or will not invite you to an in-person interview, which is naturally the next step. This is a way for an employer to filter out unqualified job applicants.
A phone job interview is a good sign that they liked what they saw on your resume and are interested in learning more about your qualifications. The phone interview is just as important as a face-to-face interview.