A related story:

A friend of mine one day started getting similar calls to his cell phone. If he answered the call, he'd hear either nothing or faint muddled noises. They were always from the same number, and calling the number back did no good; the other end never answered and went straight to a non-configured voice mail box.

These calls didn't have a set frequency, but happened at least a few per day, and sometimes several in an hour. All times of day, too.

From the number and the voice mail recording, my friend figured out it was a Sprint PCS number calling him, and called Sprint for some help. A couple of calls didn't get him any information, but finally one day he got a rep who told him that number wasn't assigned to an actual customer, but was assigned to a Sprint PCS store.

My friend immediately went down to the local Sprint PCS store to see what he might be able to find out there. After talking to several employees and having them all put their heads together, they finally stumbled upon the following:

Seems that one of my friend's friends had apparently called my friend once from one of the demo phones at this Sprint PCS store. That particular phone was fastened to its display stand with a zip tie that partially obscured one of the buttons (the talk button?), that when held down became a last number redial button. Apparently there was just enough pressure on the button that a temperature fluctuation or a breeze or something would make it actually make contact and dial my friend's phone. So, for days, this phone was just sitting in some out of the way corner of the store, making these repeated calls to my friend's phone with no one in the store even noticing.

As I recall, since my friend also had service with Sprint, he convinced them to give him some free minutes for his trouble or something.

Either way, I think your situation must be similar. Someone has a piece of equipment with a modem that needs to dial every 40 minutes and they've entered the dial up number wrong or something. Don't know why it's dialing out over a cell phone, though...