On Premise IP PBX (Audio Only) with 4 FXO ports, 4 FXS ports and supporting up to 1000 users
UCM6304A

The UCM6304A can support up to 1000 users and 150 concurrent calls and has a built-in instant messaging (IM), voice/web conferencing platform, and the free Wave App, which allows users to connect and collaborate from desktops, mobile devices, IP phones, and other SIP endpoints. 
It offers a best-in-class hybrid platform that combines the control of an on-premise IP PBX with the remote access and system manageability of a cloud solution by providing UCM RemoteConnect cloud service for distant users. 
The UCM6304A provides a robust business communication platform for any organization by providing a high-end unified communications and collaboration solution packed with a suite of mobility, security, instant messaging, voice conferencing, and collaboration tools.

Features
• Supports up to 1000 users & up to 150 concurrent calls
• Zero configuration provisioning of Grandstream SIP endpoints
• Built-in Instant Messaging, Audio Conferencing & Web Meetings platform supporting access from computers, mobile devices, and SIP endpoints
• Wave for Android, iOS, Chrome and Firefox browsers allows communication with all UCM6300 users & solutions
• API available for third-party integrations, including CRM and PMS platforms
• Advanced security protection with secure boot, unique certificate and random default password to protect calls and accounts
• Three Gigabit auto-sensing RJ45 network ports with integrated PoE+ and support NAT router
• Automated NAT firewall traversal service facilitates secure remote connections
• Compatible with GDMS for cloud setup, management and monitoring