Legal Question in Business Law in California

You are mistaken. The registered agent is simply an ADDITIONAL option for serving a corporation. A corporation can also be served by serving any officer, director or managing agent at the corporation's principal place of business or any other place an officer, director or managing agent can be found (so, e.g., you can serve a bank at any branch that names the branch manager as a vice president (almost all banks do). There may be other problems with the service, for example if the customer themselves did the service it is invalid since service must be made by a person who is not a party to the case, but there is no requirement that they use the option of serving your registered agent. Also, bear in mind that service is presumed valid, unless and until it is contested by a motion to quash service.

Mr. McCormick is right. You must accept service via the agent, but that doesn't mean he is the only person who may be served for the company.