SIP Trunking uses Internet Protocol (IP) to enable organizations to place telephone calls through the Public Switched Telephone Network (PSTN). SIP trunking technology requires either an Internet connection, or a VoIP vendor, an IP-enabled PBX, and a border element or gateway to support IP-based voice communications. Using SIP, voice becomes an application on the IP Network... like email or web is an application.
There’s no physical trunk associated with SIP, despite the fact it sounds like it may be a bulky, cumbersome technology. Instead, it’s a lightweight set of protocols that are delivered via a SIP provider enabling your organization to unlock the benefits of Internet-based phones.
While ISDN lines are physical wires, SIP trunks are entirely virtual. There’s nothing for the operator to install or for you to maintain. Everything works over your existing internet connection. As long as you have high-speed internet access and enough bandwidth, you can make and receive calls.