What an ALG changes
A SIP application-level gateway inspects signaling and may rewrite addresses or open temporary media pinholes. This can help an endpoint that does not know its translated address, but can conflict with a PBX, SBC or endpoint that already manages NAT traversal. An ALG is a specific function; disabling it does not disable NAT or the firewall.
Encrypted SIP also changes what a middlebox can inspect. A router cannot ordinarily rewrite SIP inside a TLS connection it does not terminate. An SBC deliberately terminating calls is a different architecture from a transparent router helper.
Paired observations help locate a change or loss. Account for clock offset, NAT and capture drops before attributing a difference to the network.
Collect a before-and-after comparison
Capture one authorized test call on the LAN and WAN sides. Compare the same request's Contact, Via and SDP addresses and ports, accounting for the expected IP-header translation. Record whether the phone, PBX or gateway is intended to advertise the public media address. Unexpected rewrites are evidence; the presence of a setting named SIP ALG alone is not proof of the fault.
Repeat with the documented helper disabled during a controlled maintenance window. Existing firewall state can preserve old behavior, so follow the router's procedure for recreating only the affected sessions. Do not flush all business traffic casually.
Keep the result measurable
Check registration refresh, inbound calls after idle time, two-way audio, hold, transfer and hangup. If disabling the helper breaks a required path, restore the previous setting and investigate the intended NAT design. Save paired captures and the exact router release. The useful conclusion is which component owns address translation for this deployment, not a universal switch setting.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.