Called my office phone and did not leave a message.

I've gotten a call from this number every other day, 3 calls now; different times.

Calling it back gets me a message that they're a 3-rd party verification company that verifies for free trade magazines.  Since I take a few of those, I'll answer next time, I guess.

called everyday this week and didn't leave a message.

Calls everyday without leaving a message, could be a magazine looking for a survey

In fact, it's someone calling to verify subscriptions to Electronic Design magazine; I took the call today, and the rep was quite professional.

So if you take trade mags -- especially that one -- you should probably answer.  ;-)