I have replied to many posts like this one at this site, so if you want to know what I said, you can search my history.
That said, as someone who not only is an office manager for a practice, but also works for two billing companies in my free time, I can see things from both sides. As the office manager(who is also the front desk person), there is no getting past me to the doctor. You deal with me first, then if I think we need your services, I talk to the doctor. As a billing company employee, I have seen what tactics work to get a client, and which ones don't. The quickest way I have seen companies get their first client is to actually know a doctor/someone on their staff, and convince them to use your company for their billing needs. Barring that, personal visits to drop off info is best. Cold calling and mailers are the worst, as far as I am concerned, from both ends of things.