The most common mistake an investigator can make is to go into the phone interview as if it was just a normal conversation. It is not. The smartest thing one can do is to be fully prepared before you even pick up the phone.
The first thing that I like to do is type the address where the sighting occurred into the Google Maps, and get a good look at the area. Then I take it down to Street View and move around a bit to see what the neighborhood looks like. It is extremely helpful to try to put yourself in the location the sighting occurred while reading the account. It really helps an investigator to visualize the setting of the sighting.
Next I do a legitimate background check on the witness, just to verify their personal information that they have supplied. If none of their personal information matches up, chances are their account won't either, so save yourself a lot of time and verify who they are right off the bat.
Then it never hurts to check out the website of the local news, or newspapers for the day of the sighting to see if anything might have occurred that could have been misidentified. I also always check out the weather data for the day in question before hand. That way if the witness says that it was a clear night out, and you know that it was cloudy and overcast, you have something to check and see if it might have led to the sighting. Remember, witness memory can be faulty, so the more information you have before you talk to them, the better off you will be.
When you feel that you have fully prepared yourself, then it is time to call the witness. I myself like to use Skype when interviewing a witness because it has a Call Recorder App that does a great job of recording the conversation. This is useful in a lot of cases because then the recording can undergo Voice Stress Analysis, which is just as good as a lie detector in most instances. The other advantage of using Skype is that then your witness will not have your personal phone number unless you give it to them.
It is very important to remember to always be courteous when on the phone with a witness. Keep in mind that you are representing MUFON, and be as professional as possible at all times. NEVER record a witness without their permission! Always ask permission twice...once before recording, and then again when you start recording. Make sure that you have both, you asking permission, and them giving permission on your recording. Then you will want to state your name, MUFON I.D. number, the date, and the time. Obviously there is no need to record every witness interview. You will have to use your judgement depending on the details of the case as to whether it warrants recording or not.
Having a list of questions already made up is a great tool, You also want to think of different ways to ask the same question. It's just one more way to check and see if witness statements are consistent, and the witness doesn't contradict themselves.
I like to start by having the witness give an account of what happened again verbally, to see if it matches what they wrote in the report, keeping an eye out for any differences. Try to not interrupt, but take notes of questions you will have for them afterwards. The witness might pause as if waiting for a verbal cue to continue, and it's okay to encourage them on, but avoid leading the witness at all costs! Let them tell the story...don't ever provide details for them. You also don't want to sound too excited about anything they might say, even if inside you're whooping with joy because you just discovered the holy grail of UFOlogy. It will not help the case if the witness gets the wrong impression, and then starts making the details fit what they think you want to hear.
Make sure that you have all your questions answered to your satisfaction before you end the interview. Chances are you will not be talking to the witness again, so make sure you have all of the information that you need from them while you have them on the phone. Also be sure to thank them for their assistance with the case. You do not want to give them the impression that you will be calling them back with the results of your investigation. You can let them know that they will be able to find out the case outcome on the MUFON website if they wish.