What is the main reason to check for SIP ALG when experiencing calls dropping?

When troubleshooting dropped calls, checking for SIP ALG is crucial because SIP ALG, or Session Initiation Protocol Application Layer Gateway, can indeed interfere with call audio. SIP ALG is designed to help with the transmission of SIP packets, but it can sometimes modify the packets incorrectly, leading to issues in call setup and resulting in audio problems, which could manifest as dropped calls. This interference can happen because SIP ALG attempts to rewrite the headers of SIP packets, which can lead to the SIP signaling becoming distorted or unrecognizable to the endpoints communicating.

This means that a correctly functioning voice call, which relies heavily on proper SIP signaling and audio stream management, may become faulty when a router or firewall is misconfigured with SIP ALG enabled. Thus, disabling SIP ALG is often recommended as a first step in resolving dropped call issues to ensure that SIP packets are not being altered in a way that disrupts the connection.

While other options mention potential network issues related to SIP ALG, they are not the primary concern when it comes to call drops, which are often more directly linked to audio quality and call integrity.