Network & QoS
SIP ALG
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
SIP ALG (Application Layer Gateway) is a router feature that tries to help internet phone calls pass through your firewall by rewriting their traffic. It does this badly so often that turning it OFF is the standard first fix for mysterious VoIP problems.
How it works
Your phone system talks to its provider using a protocol called SIP, whose messages contain addressing details. Years ago, router makers added SIP ALG to 'fix up' those details as calls pass through home and office firewalls. The intent was reasonable; the implementations were not. Many routers rewrite the messages incorrectly — mangling addresses, breaking the phone's registration, or corrupting the answer that keeps a call alive.
Modern phone services don't need the help — they solved firewall traversal on their own years ago — so the rewriting is pure downside: an uninvited middleman editing conversations it doesn't fully understand. That's why virtually every VoIP provider's setup guide contains the same instruction: find SIP ALG in your router settings (sometimes labeled 'SIP passthrough' or 'SIP transformations') and disable it.
Why it matters
Suspect SIP ALG first if you see any of these. Phones randomly show as unregistered. Incoming calls go straight to voicemail while the phones sit idle. Callers can hear you but you can't hear them, or the other way around. Turning SIP ALG off is a free, five-minute fix. It's the highest-value single checkbox in VoIP troubleshooting, which is exactly why it deserves a page: the symptom looks like a bad provider, and the cause is a router being helpful.
The classic SIP ALG sign is calls that drop at almost exactly 32 seconds. When a router rewrites a call's addressing badly, the confirmation that the call was answered never arrives, and the phone system gives up after 32 seconds, a timer written into the SIP standard. Calls dropping on that schedule point to the router, not the provider.
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How do I turn off SIP ALG?
Log into your router's admin page and look under firewall, NAT, or advanced settings for 'SIP ALG,' 'SIP passthrough,' or 'SIP transformations' — the wording varies by brand — then disable it and reboot the router. Every major VoIP provider publishes per-router instructions, and on business firewalls your IT person can do it in minutes. Then make a 40-second test call.
Is SIP ALG ever worth leaving on?
Almost never with a modern phone service — providers handle firewall traversal themselves and uniformly recommend disabling it. The rare exception is an old on-premise phone system whose installer specifically configured around a particular router's ALG. If nobody made that deliberate choice, off is the right setting.
Why does SIP ALG exist if it breaks things?
It's a leftover solution to a 2000s problem: early internet phones genuinely struggled to work behind home routers, and ALG was the router industry's patch. Phone services then solved the problem properly on their end, but the feature stayed in router firmware — often enabled by default — where it now mostly interferes with the fix that superseded it.
I disabled SIP ALG and calls still misbehave — now what?
Then the router did you the favor of being innocent. Work through the next suspects in order. Run our connection test for packet loss and jitter, since network quality causes most of the remaining audio problems. Check for double NAT, meaning two routers stacked between your phones and the internet. Then ask your provider to check its registration logs, which usually show exactly which part of the call is failing.