My question is if cold calling is still effective for larger businesses?

I have a target list of businesses that I am sending personalized letters to. Most are more mature businesses with###-###-#### employees. After a couple of days I follow up with a call. This is typically answered by an admin who sends it to the owner's voicemail and I never hear back. If the admin asks what my call is about I say it is regarding confidential mail correspondence we have exchanged.

I am not sure what I could do differently to improve the outcome but would welcome any feedback. I know not every business owner wants to sell but I would expect better results than I am getting. What are the best practices and how can I fine tune my approach?