Which telephone communication option should I choose between the client and the service provider?

Hi. I’m launching a business and have a question about the best communication setup to choose. We operate by referring the client directly to the worker; the worker then speaks with the client and agrees on the price. Our firm doesn't take any upfront payments; instead, we collect our fee only after the client has paid the worker for the job. I want to avoid fraud—specifically, situations where workers or call center staff might cheat me. I’d appreciate any ideas, feedback, or tips you might have.