In computing, a client is a program that makes a service request from another program, the server. This is in contrast to a “master/slave,” where one program controls another, and “peer-to-peer,” in which either of the two programs can initiate a transaction. SIP stands for Session Initiation Protocol, which is the protocol that either end of a conversation can initiate a transaction (conversation), the two ends of the conversation will have an end-to-end connection for the duration of the interaction, and then the end-to-end connection will be torn down. A SIP client is any network device that sends SIP requests and receives SIP responses. SIP clients connect to SIP proxy servers.
The purpose of a SIP client is to establish real time communications (RTC). Because of this, SIP clients contain softphone functionality, and usually some other features. For example, a SIP client may also be able have video capabilities, chat, file transfer, and even allow remote desktop access. Some SIP clients are open source, and free to the average user, and others are proprietary. Still others are exclusive to a particular provider, such as a business VoIP provider. What makes SIP such a valuable tool is that it is not tied to any particular service. 3CX, Bria, and SIPDroid are three of the most popular SIP clients, and they are recommended by many of the business VoIP providers that we review. A SIP client can be a desktop client or a mobile phone client, although most developers release both desktop and mobile clients.
Configuring a SIP client is easy. In fact, many clients have accounts pre-configured to popular VoIP providers. Click/touch the “add account” button and add your provider (if not listed), and then add your user name, which is your SIP address or your phone number, and enter your password, and then your domain, which would be “sip.example.com.” Depending on the SIP client you can have one or more than account on the same client—paid SIP clients allow more than one account, generally. You can also set a SIP client within a PBX system. What that means is you can dial your colleague’s three- or four-digit extension right from your smartphone, and the phone will recognize it as an extension within your own PBX. Once you have a free SIP client, you can make free SIP-to-SIP calls.