If you have any open job applications, you never know when a potential future employer will try to get in touch, so it's usually best just to let any calls from unknown numbers go to voicemail.
Pretty much nothing can go well by getting caught off guard by a call from a potential employer. If you're at work, you may need to awkwardly find a private room. If you're at lunch or out with friends, the background noise from your end of the call could sound distracting or unprofessional. Even if you're in a perfect position to take the call, it's still probably better to glean some information about the caller from a voicemail and get yourself in the right frame of mind before calling back.
It may seem obvious, but it's easy to answer calls immediately out of habit, so it's a good tip to keep in the back of your mind. Be sure to check out the source link for more pre-interview tips.
Are You Blowing the Interview Before It Even Starts? [US News & World Report Money]