Most every company has a process they follow for interviewing. Many times you will start off with a phone interview. This makes it easy for the company to weed out bad candidates without wasting a lot of time.
In obtaining one of my previous jobs, I had two phone interviews with the HR representative. The first one was to understand my situation and the second was to better get to know me and my skills. Even in the short phone conversations they were assessing whether or not I was a good candidate and if I would be a good fit for the company.
If you get a phone interview, it means they are interested enough to spend more of their time learning about you. This is usually done during a weekday, so you may have to do it on your lunch break. The place you choose should be quiet and if you use a cell phone be sure to have good reception. It is getting harder to get access to a landline phone, but if one is available it is better for an interview since the reception will be much clearer. If you are on a cell phone be sure to mention that at the beginning of the interview so the interviewer will be more understanding if there is bad reception. You also want to be in a place where you would not be distracted.
In both cases where I had phone interviews I had to do it from my cell phone since I could not find easy access to a landline phone. In one case I went to one of those gas stations that have tables and chairs for eating. It was very quiet and undistracting there. In another interview I went out to my car in the company parking lot and sat in the driver seat to talk to the interviewer during a break.