i dont know it is STUN or SBC.
Please note that i cannot find the specific model in Fanvils website and i am sure they are not included in the supported 3CX devices so i am not sure how you provisioned these devices. I would recommend using 3CX supported devices however to ensure compatibility.My fannvil is IP7921, i dont know it is STUN or SBC. This happen when i calling from one extension to another. Firewall check is pass. my version is 15.5.0
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