Business professionals: Do you have any tips or tricks to get people to respond to your communications?

I deal with a lot of attorneys and their assistants for my work. Whatever the reason, many of these people do not respond to communications. I call and leave messages, I fax, I email. I do any combination of the 3 multiple times and still no response.

Anybody have any suggestions as to how I can get these assholes to contact me? Really. Sometimes it takes weeks and my boss gets irritated with me. As if it’s my fault.