Go to the website call2friends, and type in your cell phone number. That number will come up. It's an internet to phone call making service. It's probably someone you know trying to get a hold of you.
Unfortunately, this site (call2friends) is becoming infamous as a mechanism for prank calling. It allows you to make unidentified calls via internet. I would be very suspicious of a "friend" who finds it necessary to hid their identity from me when calling.