Starting a collection agency/service is a bit more involved than a medical billing company. First, you may not be able to charge % of revenue depending on state laws. Next, there is the FDCPA which has to be followed TO A T. In some states, you may even need to be bonded and bonds can be pretty expensive. There are even states that require the bond even if you are not collecting the money but just merely acting as a collection agency requires a bond.
Again, a lot more involved. Then there is the biggest hurdle, doctors when turning over accounts usually look for collection agencies that have been in business a while so it can be really hard to break in a new business.