Typically, an employer conducts a telephone interview or phone screen if the prospective employee doesn't currently live near the job site or if there a lot of candidates for a job.
This will allow the employer to determine whether someone is a viable candidate before investing the time and money involved in arranging an in-person interview. An employer may also conduct a phone interview if the prospective job involves a lot of telephone use.
You should treat a phone screen like an introductory interview. Try and make sure you are in a quiet room with a door that can close. Answer the questions as though you were in front of your perspective employer and come up with as many specific examples and anecdotes as you can. This phone call will determine whether or not you advance to the next step!!