If your VoIP calls are dropping unexpectedly, cutting out mid-conversation, or struggling to connect at all, there's a decent chance something called SIP ALG is causing trouble.

SIP ALG stands for Session Initiation Protocol Application Layer Gateway. It's a security feature built into virtually every modern router, designed with good intentions: help VoIP systems work smoothly by managing how your phone calls navigate your network's firewalls. The problem is that many routers implement it poorly meaning it does the opposite, creating the very problems it was meant to prevent.

Why SIP ALG exists in the first place

To understand why this feature exists, picture what happens when you make a VoIP call. Your phone sits on a private network inside your office or home, operating on what's called a Local Area Network (LAN) IP address something like 192.168.1.50. When you call someone, that data needs to travel across the internet to reach the Agilico Connect Cloud servers. Your router performs a trick called Network Address Translation (NAT), which essentially translates your private IP address into the public one that the outside world sees.

Here's where the complexity starts, VoIP works differently than regular web browsing. It uses a protocol called SIP (Session Initiation Protocol) to negotiate the connection, and within those SIP packets is all sorts of information about addresses and ports. The problem is that these packets contain your internal private IP addresses. When they leave your network, they're suddenly invalid the VoIP provider on the other end has no way to reach an internal address.

Enter SIP ALG. Its job is to inspect these packets as they leave your network, translate those internal private IPs to your router's public IP address, and dynamically open ports to let the call data flow in both directions. In theory, it's a helpful traffic cop managing a complex intersection.

Why it often breaks VoIP

The theory sounds great. The reality, unfortunately, is messier. Many router manufacturers and there are lots of them implement SIP ALG in ways that are either outdated, incomplete, or simply buggy. Instead of just fixing the IP addresses, these implementations can unexpectedly rewrite data, corrupt SIP packets entirely, or route your voice traffic to completely wrong ports.

The result is a cascade of VoIP problems that appear baffling at first:

• One-way audio: You hear the other person fine, but they can't hear a word you're saying. It's frustrating and makes calls nearly impossible.
• Calls dropping immediately: You answer the call, and within seconds or sometimes instantly the connection dies.
• Registration failures: Your phone randomly disconnects from the system throughout the day, or incoming calls go straight to voicemail because your phone isn't registered to receive them.
• Disabled call features: You can't put calls on hold, transfer them to colleagues, or retrieve parked calls. The technical features your business depends on simply stop working.

These symptoms are frustrating enough that many people assume something's catastrophically wrong with the Agilico Connect service. Often, it's just SIP ALG doing what it was poorly programmed to do.

How to fix it: disable SIP ALG

The recommended troubleshooting step for VoIP reliability issues is almost always the same: turn SIP ALG off. Every router is different, so we'd recommend raising a support ticket with your IT provider for specific guidance on your equipment. That said, the general process is fairly straightforward on most devices.

1. Check your router settings: Log into your router’s administrative dashboard. Look under headings like Security, Firewall, NAT, or Applications/Gaming.
2. Turn it off: Locate the SIP ALG (or sometimes "SIP Passthrough" or "Conntrack SIP ALG") toggle and disable it.
3. Reboot: Restart both your router and your VoIP phones/softphones so they establish fresh connections to your provider's network.