SIP (Session Initiation Protocol) is a protocol used in VoIP communications allowing users to make voice and video calls, mostly for free. A SIP client is a program that you install on your computer or mobile device. It contains softphone functionality and some other features and provides an interface for you to communicate. There are different types of SIP clients. Among the most common ones are the applications offered free by VoIP service providers, to use with their VoIP services. Some of them support SIP. But you have clients that are built for SIP and do not depend on any service. You can use them with any SIP account, and can even use them within a PBX environment. Here is a list of the most popular free SIP clients on the market., and here is how to configure a SIP client.