Telephone marketing is a tried and true staple of real estate marketing. But the "good ole days" are gone. Learn the DNC (Do-Not-Call) Registry Rules and abide by them, or you may pay dearly.
Understand the Do Not Call Rules Yourself
The DNC rules aren't complicated, but they are vague in some areas that haven't been tested yet for real estate. Read and understand the rules and interpret them on the conservative side. At $11,000 per violation, it wouldn't take long to use up any commission income derived from aggressive telephone practices.
Develop DNC Procedures and Training & Enforce Them
Don't count on the Independent Contractor status of agents to get you off the hook for their actions. This may not have been tested yet, but it's highly likely that the government will hold brokers responsible for the telephone marketing practices of their agents. Develop a Policy Manual with clear guidelines and training, or at least an orientation, that makes the rules clear to your agents. Record their attendance with a signature of understanding.
Don't Rely on the Safe Harbor Provisions
The procedures and training are specifically mentioned in the Safe Harbor Provisions of the DNC act. However, recent enforcement actions have shown that the government is very strictly interpreting what is allowed to be an "inadvertant error".
The only safe approach is to have a strict policy, train your agents on it, and enforce it.

