Do you mean collecting social security is voluntary because paying the tax never has been voluntary. There was no lawsuit filed in the Taco Bell case.
http://www.curiousevidence.com/(S(gdoih5r3mezdk1htcrdhfwvx))/samples.aspx?id=23
http://www.ssa.gov/history/InternetMyths.htmlhttp://www.factcheck.org/2009/03/fdrs-voluntary-social-security/