Provide a landline if possible, it comes across better on both sides and is a lot less echo-y than a mobile. If you have no landline access in halls, sometimes careers services will give you access to a room with a phone at the time you need if you ask.
It's all about your voice
The main thing to remember with a phone interview is you have only one means of conveying what you want to say and who you are - your voice. In a face-to-face interview, you have hand gestures, head nodding or shaking, posture of your body, your clothes, your eyes, your smile, your laugh. It is vitally important that you have that in mind before you start.
So, now that I've made you very self-conscious about your voice all of a sudden, how do you control it and what is it about the voice that can make or break the interview? One of the first and most difficult things is coming across enthusiastic and confident. i.e. not nervous to the point at which your voice goes unnaturally low / unnaturally high. Your voice will contain tones and enthusiasm if you try to speak as naturally as you can. Easier said than done, I know, but it's something worth thinking about. The last thing you should have in your mind before the phone rings is 'I am a strong candidate for this job - I would be happy to get it and they would be lucky to have me'. If you believe it, it will come across in your voice. Some people even recommend standing up as they say it comes across stronger. I'm not too sure - I think it can also make you a bit less relaxed and you might even start pacing.. but it's worth considering if you think it might work for you.
Most large companies now have recruiters write down what is said at interview - sometimes in full. Keep that in mind when pacing yourself - it doesn't mean you should go slower than you normally do, but try not to go too fast as nerves can often cause that to happen and speak as clearly as you can. If there are any silences when you have finished your answer, they might well be finishing writing your answer down, so don't feel that you have to keep talking to fill the silence. If you have answered the question, stop. If they want you to give more detail they will say 'go on', or 'can you expand on that'.
Use of notes
Obviously being a telephone interview, you can have any notes or information you might need in front of you. My advice is keep it fairly minimal. If you have too much in front of you it is easy to get flustered looking for information to distract you from the phone call. I recommend having very well organised papers, laid out side-by-side, so that you aren't shuffling paper around whilst on the phone. I don't recommend writing answers verbatim down, because it is too easy to start reading them out and it comes across that that is what you are doing. Even worse, sometimes you might have an answer written down that you hope to use which is an answer to a question that is similar to a question they asked, but not the same - and so the temptation to use it will result in you not really answering the question at all. Write down key bullet points that help you tell the story and will help you ensure that you do not forget any major parts of it.
It is very important that you stick to the "STAR" model, or something similar, when answering competency questions on the telephone (Situation, Task, Action, Result). It will keep you on track and make sure that you are addressing the question. Make sure that in closing, you link back to the exact question asked so that you leave them in no doubt that you have addressed what they asked you.
To go back to my point about telephone interviews being tough.... most other recruiters I'm certain agree with this. It's tough for you, and its also tough for us. So during the telephone interview you should be prepared to ask a couple of questions as you go along about whether you are giving them what they are looking for. After you finish your first competency question ask "was that enough detail for you, would you like any further information about that?"; if you are unsure about whether an answer has fully addressed what they are looking for, you can ask "have I answered your question on that?". Note, I'm not saying that you need to do that after every question, but for the first couple of competency questions where you are trying to figure out how much detail they want.
Companies ask a different number of questions in that 20-30min interview - anything from 3 questions to 15 questions. Obviously that will affect how long your answer is. It might be worth asking at the start for an indication of how many questions they want to go through in that time - you could say "could you give me an indication of the likely number of questions so that I know how much time to spend on each answer?" or something to that effect.
They will invite you to ask any questions at the end. Keep it to no more than 1-2 ( even if the recruiter keeps saying 'any more questions?'). Ask something that would help you at the next stage. There will be plenty of time to ask more questions if you get through, you dont need to ask them all now.
Incase they want to capture your availability for assessment centres - a possibility - make sure you have an awareness of any exam/holiday dates that you have coming up so that you can advise them.
In closing, having said it's all about your voice, I would caution against too much humour - jokes or sarcasm don't translate very well on the phone - especially with no body language to accompany them. I'm not saying don't build a rapport - definitely do this as best you can - but take care with sarcasm in particular.
'Tis the season for telephone interviews at the moment - good luck all
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