Using Siri for commercial purposes?
Since Apple doesn't offer an API to use their Siri services, you might be limited to coming up with some Rube Goldberg machine that presses the home button of your iPhone and rubs a buddha belly for luck (or whatever else you desire to have happen simultaneously with the incoming call) so that Siri will work at the exact moment your caller is speaking to her.

Again, there is no API to do anything except listen to what Siri responds sonically so it's not clear how this might be useful for anything but a fun youtube video.

Now, your business might very well be able to license a voice like Siri (or the same exact voice for enough $$) by contacting Nuance or some other computer speech vendor - but their product likely won't run on iOS since other custom hardware is likely to be more capable for an automated PBX.