In interviews with National Review Online, Koch executives responded to the incident and pledged to “not stop” supporting free-enterprise initiatives, even as opponents attempt to sully the Koch name and the groups that brothers David and Charles Koch, the company’s co-owners, support. They also noted that David Koch and the governor have never met or spoken.
That would explain why Walker wasn't aware he was taking a prank call -- it's kinda difficult to recognize a voice you haven't heard.
Robert Costa at National Review Online notes that Koch Industries isn't very happy about this for other reasons, too:
Koch executives are not happy about the use of David Koch’s name by the blogger. “It was a fraudulent call,” says Mark Holden, the general counsel for Koch Industries. “There are serious fiscal issues at play in Wisconsin. Yet our opponents are interjecting us falsely into this story. But our Wisconsin story is about bringing and keeping good manufacturing jobs in the state. It is disturbing that when a blogger calls using the Koch name, it is used as an opportunity to attack the company.”
Well, that's what you boys get for speaking up. Ask Target about it -- they gave some money to Tom Emmer and they were suddenly anti-gay and a bunch of other things. Never mind that Emmer, much to the consternation of some of his allies, never talked about social issues during his campaign. Doesn't matter. You just need to shut up.