There's a couple of techniques you can use, one of my favorites with a customer that I've already met is below.
1) The next time you get the dreaded voice mail, leave them this message. Hi this is Mr. So & So with XYD company, I tried to reach you on several occasions, I am going to be in your area next week. What I'd like to do is to stop by at 1PM on Wednesday the 4th (you set the date) to meet with you, if this is not convenient for you, please call me at (phone number), other wise I'll see you then.
2) Send them a letter, yes a letter, however don't send it regular mail, reach deep into your pocket and spend the extra cash to send it priority mail or fedex. The letter will get opened, be specific with how you want to move forward and have call scheduled for the day after of the day the letter arrives.
3) Ship them or drop off a manufacturers promotional item with a letter inside of the box, it could be golf balls, a desk set or a nice pen. You gotta spend money to make money right?
4) Try contacting them via social media, such as linkedin (which I like best), facebook or twitter, a couple a recent articles stated that messages sent via linkedin get a better response that regular email.
5) Contact other business owners you know in the same business and see if they know the person you are trying to connect with and ask them for an introduction either through social media, a letter or a phone call.