There are several available types of CTI, but the two most common options are Desktop and System. The Desktop system (AKA first-party telephony) is when the IP PBX telephone system connects to your desktop computer, and vice versa.  The System option (AKA third-party telephony) uses a central server that connects to your phone system -- meaning there isn’t a direct connection between the computer and the phone. Instead, everything is connected via the Local Area Network (LAN.)