Taxes, fees, and surcharges for your mobile service are based on a variety of factors, including local tax rates and the address listed on the account. For this reason, state and local jurisdiction may apply fees unique to your mobile bill, including any emergency cellular service fees, such as calls to 911.
Among these fees, you may notice the Federal Universal Service Fund (USF) charge on your bill. This is a fund designed to provide service to rural and low-income areas.
For more on reading your online bill, check out this article.