ITSP = Internet Telephony Service Provider
Carrier or Internet Service Provider who offers packet-based Voice over IP service via the Internet. Wired network access is provided via xDSL or cable modem. Mobile access is possible via WLAN or UMTS. The service may be offered independant from and in addition to the physical Internet access.
The ITSP provides usually free of charge calls within his network and gateways to public switched and mobile networks with cost.
See also Collaboration with VoIP Providers.