The number for each state's child support office can be found on the contacts page of the website for the Office of Child Support Enforcement. The site also provides the local addresses and fax numbers.
It lists contact information for all United States territories, including tribal child support agencies and commonwealths. This is a government organization under the umbrella of the United States Department of Health and Human Services. It doesn't work directly with parents, but provides informational resources. The OCSE partners with local child support agencies to ensure they are following and enforcing policies to secure financial stability for all minors.