The Grandstream UCM6304 is an easy to manage device that does not require licensing fees and can support up to 2000 users and up to 300 concurrent calls. IP PBXs allows businesses of all sizes to build powerful and scalable unified communication solutions in an easy-to-manage fashion with no licensing fees Analog Telephone FXS Ports: 4 RJ11 Port PSTN Line FXO Ports: 4 RJ11 Port Users: 2000