Engineers need more than just their resume to display their skills to job interviewers; they also need to vocally present themselves to possible interviewers as worthy of eliciting a job interview at the company in the first place. Job interviewers tend not to schedule personal appointments with every single person hopeful to land a job, so an engineer needs to rely on contacting an interviewer over the phone to show that he or she stands out from the crowd. Conducting an interview via telephone is an important step toward finding work at an engineering agency, but it is nonetheless a prelude to the actual interview that one brings a resume to.
The importance of an advance interview conducted vocally over the phone makes preparing for it critically important in its own right. Engineering job placement is a tricky task, and takes time and effort by both parties. Your first order of business would be to find out who among the company would be speaking on the other end of the line so that you can write and and practice reciting your questions with that speaker's profession in mind. That way, you can avoid sounding overly technical to someone who may not be a manager involved in the company's engineering work. Your questions should not be so restrictive and specific in scope that your interviewer would think you have not bothered to learn about the company's business and needs as a whole.
It is vitally important that you will not be interrupted or distracted on your end once you begin your conversation, so you should make sure that you schedule your interview on a day where you will not be interrupted at your own house. None of your other phone devices or electronics should be capable of audibly sounding off, and your windows and doors should be shut tight. Granting these, have printed documentation within your arms' reach that lets you easily refer to your resume details and other important information you might otherwise forget in the heat of the moment. These will help you adjust your previously planned answers for whatever the phone interviewer will ask you about.
Because this is not an interview taking place between two people facing each other across a desk, you need to be capable of conveying through your voice and tone that you, the job seeker, are interested and eager in the position. Nonetheless, you need to keep composed as if you were in an actual interviewer's office trying to show yourself as a professional with an appropriate state of focus. It can help to take notes recording important points the interviewer says, but writing them by hand may be preferable to typing them because it will be less distracting in terms of noise.
By the end of the interview, express further interest in your potential future position and ask about how the company would conduct its following interviews with you. Within a day of the phone interview, send a formal email to the phone interviewer to convey your thanks for the time they gave you.