CLEC vs. ILEC
CLECs and ILECs are two different classifications of phone companies, distinguished simply by the year in which they began providing phone service.
ILECs (incumbent local exchange carriers) are phone companies that were conducting business prior to 1984. That is not an arbitrary year or a reference to the book, but rather the year when AT&T was split up into seven RBOCs (Regional Bell Operating Companies). These "Baby Bells," as they're more commonly known, and their four successor companies are now known as ILECs.
Verizon is an example of an ILEC, and so is Bell South. They are America's established and well-known phone companies.
OK, so what is a CLEC? A CLEC is a competitive local exchange carrier. They arenewer companies who offer services in areas previously monopolized by one or more ILECs. Some CLECs do have services which directly overlap the ILECs, for example, in the case of a CLEC who provides internet access and phone service to retail customers. Other CLECs simply provide transport across networks and don't sell anything to customers outside of the telecommunications industry.
ILECs are required by the government, in the spirit of fostering competition, to provide service and fair pricing to CLECs, who then offer their service to end users. Of course, not all CLECs need that. Some buy almost nothing from ILECs but still compete with them.
CBeyond is an example of a typical CLEC. They sell phone and voice service to business customers using their own equipment and lease phone lines from ILECs. An atypical example would be Level 3, the largest CLEC. Level 3 is a major player in the worldwide telecommunications market and controls one of the largest IP transit networks in the world.