My clients and I have found that a 40 minute phone screen is a great way to make the interview process more efficient. Candidates appreciate them as they can learn more about the opportunity directly from the hiring company prior to taking time off from work to formally interview. From the client perspective, 3-6 people do not have to take time out of their day to quiz the person - avoiding hours of lost productivity. Using phone interviews in your screening process is a great way to narrow the field and select which candidate(s) you really want to pursue.
I've had to conduct plenty of phone screens over the years, and I've found the following practices most effective:
1. Follow a 5-30-5 rule -- that is, spend 5 minutes talking about your company and the job opportunity, 30 minutes asking questions, and 5 minutes answering the candidate's questions. That first 5 minutes is a nice way to relax the candidate and get him or her excited about the job. Part of the process is "selling" the candidate on the job, of course!
2. Do not exceed 40 minutes. In other words, stick to that 5-30-5 rule. Yes, this is redundant, but it's important! The phone screen exists to make the process more efficient, not less; so if you're spending more than 40 minutes, you need to speak more concisely and choose your questions more carefully.
3. Be prepared to pass. If you are indecisive and would feel uncomfortable with terminating a candidacy unilaterally, you should not be conducting phone screens; instead, ask a coworker to do it or have a coworker on the call with you to "train" you. By the end of the 40 minutes, you should have a clear idea in your head about whether or not to proceed.
4. Standardize the questions. This is especially important for cases where several people on a team may be dividing phone screen responsibilities. To ensure the bar is set consistently, come up with a standard set of questions to ask during the phone screen -- and organize them by level of difficulty. The questions should be specific to the job, of course. This doesn't mean you have to ask all the questions or cannot deviate from the questions during the call, but for an appropriate baseline at least some of the standardized questions should be asked.
5. Test the questions. Ask coworkers holding the targeted job - or a similar one - the questions. You may be surprised to learn that the questions are too hard, or better left to an on-site interview setting with a whiteboard nearby. Vetting the questions before a phone screen will give you confidence. If the candidate falters, you'll know whether it's because the question is difficult or because the candidate is lacking a critical skill.
6. Go in prepared -- know the candidate. Take the time to look over the candidate's resume (and portfolio, if applicable) before the interview. A quick Google or social network search may add some color. Use this prep to focus the discussion. Make sure you ask the subset of standardized questions that will verify some of the claims on the resume.
7. Have a question that demands someone think on their feet. This is not to trick the candidate, but you want to ask a question for which they aren't prepared. Make it a simple question that demands a simple answer. For example, "What would you do if you worked here, and after exhaustive internal debate and discussion, you received completely different direction from 2 of our most senior leaders?" There might not be a right or wrong answer, but it will give you some insight into their values and ability to navigate difficult situations (I believe the best answer is "What is in the best interest of our customer")
8. Write down your thoughts during (or immediately after) the call. The paper trail is important so you can communicate the results with others, especially the recruiter/HR rep suggesting candidates. The feedback will (ideally) help them improve the candidate pool over time. Don't wait until later when your memory of the conversation will not be as vivid. Come up with a standard form that you and others can use to quickly record your thoughts on areas such as technical competence (score of 1-5, with optional comments), communication skills (1-5), engagement (1-5), culture fit (1-5) and perhaps experience (1-5).
9. Do not end on a negative note. If you are excited about the candidate and would like them to move forward in the process, feel free to say so. But do not do the opposite when you are not excited about them. Leave that task to the recruiter/HR -- it's their job. Otherwise you may find yourself spending extra time trying to justify your decision to the candidate. Instead, end with a thank you and indicate the recruiter/HR will follow up with them on next steps shortly.
10. Never share your interview questions with recruiters/HR sourcing the position. While often well-intentioned, I've found recruiters tend to tip off the candidates if given the chance. Their incentives, after all, are based on these people getting hired! This also means keeping the feedback generic. Do not tell a recruitier that a candidate couldn't name an HTML tag to represent a button, for example; just indicate that the candidate did not meet the minimum level of technical proficiency. The message here is don't let recuiters/HR listen in on the call.