An incumbent local exchange carrier (ILEC) is a local U.S. telephone company that was in existence at the time of the break up of AT&T into the Regional Bell Operating Companies (RBOCs), also known as the "Baby Bells."
The ILEC is the former Bell System or Independent Telephone Company responsible for providing local telephone exchange services in a specified geographic area.
GTE was the second largest ILEC after the Bells, but it has since been absorbed into Verizon, an RBOC. ILECs compete with Competitive Local Exchange Carriers (CLEC). When referring to the technical communities ILEC is often used just to mean a telephone provider.