An organization, such as an ISP or telephone company (CLEC, LEC, etc.), that supports IP telephony. Using the Internet as the primary backbone, it allows customers to make phone-to-phone calls or PC-to-phone calls. The majority of customers use an ITSP to save money on international calls, and the quality can vary substantially due to the inconsistency of the Internet. The ITSP uses IP gateways to convert between voice and IP packets.