Using an automatic dialing machine to call cell phone numbers is illegal. Making a sales call to a cell phone number is not illegal. The logic for this is simple - if the company that services my air-conditioning calls me to see how I liked their recent installation (and I did), would that be considered a sales call? And if sales calls were illegal, would they be breaking the law?
But before you grab the list of all of your clients cell phone numbers and start calling them to make a sales pitch, consider what people think. In addition to hating getting a seemingly random sales call on their cell phone, most people think it's illegal. And if you are doing something that is perceived as illegal, and certainly not very nice, you will damage the reputation of your company. Don't do it.
I hope that this helps.