If you have followed the advice given so far, there will not be any surprise interviews. By building your brand, materials and planning out your career, you will always be prepared. It will come as a second nature. Just as how comfortable you would be talking about any other aspect of your life.
A surprise interview can only take place when you have let your guard down and are not prepared. They can also be disastrous to a candidate who is caught this way because they generally are not going to give their best answer or performance, and thus, are more prone to making key interview gaffs that can knock them out of contention. Do not let this happen to you.
The most common surprise interview is the phone call; your phone rings and you indiscriminately answer it without having any knowledge who is calling you. As a result, you are now in a position to react, not act. You have just given the interviewer the upper hand while putting yourself at a great disadvantage.
As previously covered, turn the call around to your favor by saying you are not at liberty to speak at this moment but would like to call back at a time when you will have the time and privacy to talk further about the opportunity. Agree on a time and make sure you fulfill your commitment to it. This way, you have time to properly prepare yourself and put yourself in the most advantageous seat possible.
Likewise, if you do not recognize the number and allow it to go to voicemail, then you can follow the same procedure as previously advised. Follow the guidelines provided under the Screening Interview section and you should be well equipped to handle this sort of situation.