A service offered by employers called Health Advocate is a call-in center that helps customers find the right doctor, haggle over insurance coverage, and manage other medical system headaches. Health Advocate is one of a growing number of U.S. companies offering advocacy services to medical consumers. Revolution Health, a Web-based medical consumer services company overseen by AOL co-founder Steve Case, has been considering getting into the same business. The health advocacy business may be on track to become a $1 billion industry based on the demand for the service, experts say.