What are OTC (over-the-counter) bitcoin brokers?

Could you elaborate on what OTC bitcoin brokers are and how they operate within the cryptocurrency landscape? As a financial professional, I'm interested in understanding how these brokers facilitate bitcoin transactions outside of traditional exchanges. Do they provide a more direct and personalized service? How do they ensure security and trustworthiness for their clients? Additionally, are there any specific advantages or disadvantages of using OTC bitcoin brokers compared to traditional cryptocurrency exchanges?