You've got VoIP. You followed the setup instructions. Everything looked fine during testing. But now calls are dropping, audio is one-way, and phones keep losing registration for no apparent reason. You've called your VoIP provider three times. They ran tests. 'Everything looks fine on our end,' they said, which is exactly what you didn't want to hear.
I'm going to save you the fourth call. The culprit is almost certainly a three-letter setting sitting quietly in your router: SIP ALG. And it's been sabotaging you this whole time.
What Is SIP ALG, Exactly?
SIP ALG stands for Session Initiation Protocol Application Layer Gateway. It's a feature built into most consumer and small-business routers — including many from Netgear, Linksys, ASUS, and others — that was designed to help VoIP traffic navigate NAT (Network Address Translation).
The idea was noble: SIP packets contain IP address information inside the packet payload, not just the header. When NAT changes your internal IP to your public IP, it changes the header — but SIP ALG was supposed to also update the payload so everything matched up. Great concept. Terrible execution.
Why It Almost Always Breaks Things
The problem is that modern hosted VoIP platforms already handle NAT traversal themselves — using techniques like STUN, TURN, and ICE. They don't need SIP ALG's help. And when SIP ALG jumps in anyway and starts rewriting SIP packet payloads, it usually does it incorrectly, creating mismatches that confuse the VoIP server.
The results look like this:
- ▸Phones register successfully but then randomly drop off
- ▸Calls connect but audio only works in one direction
- ▸Calls drop after exactly 30 seconds (a classic SIP ALG symptom)
- ▸Incoming calls ring but the caller hears nothing when you answer
- ▸Random registration failures with no clear pattern
How to Disable It
The fix is straightforward: turn it off. The exact steps depend on your router, but here's the general path for the most common brands:
- ▸Netgear: Advanced → WAN Setup → uncheck 'Disable SIP ALG'
- ▸ASUS: WAN → NAT Passthrough → set SIP Passthrough to 'Disabled'
- ▸TP-Link: Advanced → NAT Forwarding → ALG → disable SIP
- ▸Ubiquiti/UniFi: No SIP ALG by default — you're already good
- ▸SonicWall: VoIP → Settings → uncheck 'Enable SIP Transformations'
After disabling it, reboot your router and re-register your phones. In most cases, the symptoms clear up within minutes.
Intecha's TakeWhile you're in your router, also increase the UDP session timeout to at least 300 seconds. Some routers close UDP sessions too quickly and drop SIP registrations in the background — this is the other most common cause of random re-registration failures.
What If Disabling It Doesn't Fix Everything?
SIP ALG is the most common culprit, but it's not the only one. If you disable it and still have issues, the next things to check are firewall rules for RTP ports (UDP 10,000–30,000), whether your phones are on a dedicated voice VLAN, and whether QoS is configured to prioritize voice traffic.
If all of that sounds like a lot — that's what NeedIT is here for. A VoIP Network Assessment covers all of it, and you'll walk away knowing exactly why your calls have been behaving the way they have. No more guessing.