After the Telecommunications Act of 1996, Incumbent Local Exchange Carriers (ILEC) is what the telephone monopoly companies became after deregulation. Prior to this, monopoly companies were the sole providers of phone service, and they owned and controlled the Public Switched Telephone Network infrastructure throughout the US. Afterwards, they were divided into regional companies (ILECs) and required to lease public switched telephone network components to newly formed, competitor phone companies also known as Competitive Local Exchange Carriers (CLECs). This allowed customers options to choose who their service provider would be.
Internet Service Provider (ISP) is the company you purchase your internet access from. Most IPSs provide additional internet services such as website hosting, email, etc. Some ISPs bundle internet service with other services, such as television and telephone service. As the internet has grown, the industry has steadily consolidated and is now dominated by a few large regional ISPs such as Comcast, Verizon, etc.
Internet Telephony Service Provider (ITSP) is the standards industry designation for a business that sells VoIP services. ITSPs are not carriers (ILECs or CLECs). PCS VoIP for example is an ITSP. The service is based on local telephone and VoIP techniques, and thus it is facilitated by the Internet.
Note: The FCC uses the acronym “ITSP” for Interstate Telecommunications Service Provider, which refers to a company that sells multi-state phone services via the PSTN. The FCC categorizes businesses that sell VoIP services (such as PCS) as Interconnected VoIP Providers.
