What might be a potential culprit for audio issues despite a green indication from the 3CX Firewall Checker?

A common cause of audio issues in VoIP systems, even with a green status from the firewall checker, is SIP ALG (Application Layer Gateway). SIP ALG is a feature found in many routers and firewalls that is intended to help with the handling of SIP traffic. However, this feature often causes more problems than it solves since it can alter SIP packets and potentially disrupt the signaling process needed for establishing and maintaining calls.

When SIP ALG is enabled, it may modify the headers of the SIP packets, which can lead to issues with audio transmission, such as one-way audio or dropped connections. Even with firewall settings appearing to be correct and a successful firewall check, the presence of SIP ALG can lead to unexpected audio issues in a VoIP setup.

Recognizing SIP ALG as a potential culprit is crucial for troubleshooting audio problems. It is commonly recommended to disable this feature on the router to resolve such issues, assuming that other aspects of the network configuration are correctly set.