What are the mysterious phone calls I receive every day?

Almost every morning, between 10AM and 12PM, I receive a phonecall from a number which appears as “unknown” on the screen of my iphone. When I answer, I hear a series of beeps, and then a hangup. When I don’t I get a message of about 20 second of beeps (approximately 6 beeps per message). What is this? My father said it might be a fax machine. But every day? To a cell phone? That just seems weird. This has only been happening since I got my iPhone in December.