The UC Server IP needs to be whitelisted on the UCM.
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The Grandstream UCM 63xx on-premise IP PBX appliance combines enterprise-grade voice, video, data, and mobility features in an easy-to-manage solution.
Firmware Requirements (for new installations):
Tested with 126.96.36.199
Grandstream - GXV3380, GRP2613, GRP2614, GRP2615 and GRP2616