SIP ALG is a network security protocol found in many gateway devices such as routers, firewalls, and modems. It functions by detecting and terminating continuous streams of data flowing in and out of your local network.

Why is SIP ALG Harmful to VoIP?

VoIP technology relies on a constant connection between your device and the web server where the system is hosted. This means your phone is always talking to an outside destination.

SIP ALG sees this as suspicious because it looks like the same thing what would happen if your network was infiltrated by a bad actor. It can’t know what the outside source is, what kind of data is being sent, to where it is being sent, and whether it is safe or not, so it simply blocks the transmission.

Why does SIP ALG Need to Be Disabled?

Blocking VoIP transmission can result in a variety of negative effects. The most likely effect is that your VoIP device cannot register to the hosted system at all. In other words, it can’t talk to the system and tell it where it is so it can receive phone calls. This is likely to be the case if you see “no service” or something similar on your device screen and you can’t make or receive calls.

If the device was registered before, it may stop registering or register only intermittently. SIP ALG may also cause call quality issues like dropped calls or choppy audio by terminating only certain transmissions without causing complete registration loss.

Many residential gateways have this protocol enabled by default. It is by far the most common cause of call quality issues while working from home on local internet. In some cases, it may also be present on a business gateway.

Why is SIP ALG So Common?

Most ISPs (internet service providers) have their own proprietary communication systems. They do not want you to be able to use a third-party service over their internet connection so that will be forced to pay for their service instead. For more information about why using ISPs for your business communications needs is a bad idea, click here.

While they cannot legally stop you from doing so, companies like Spectrum and Frontier can make it difficult by packaging their equipment with SIP ALG by default and labelling it as a “security measure”.

While it is technically true that SIP ALG acts as a security measure, it does so in a rather clumsy and unnecessarily overprotective way. A true security solution would be able to elegantly authenticate the source, recognize that it is not a threat actor, and allow the VoIP registration, all while blocking other data streams that actually pose a threat.

Fortunately, this is exactly the case with many third-party security solutions such Zyxel. For more information about why ISP devices are insufficient security solutions for VoIP businesses, click here.

What Can Be Done About SIP ALG?

Fortunately, most gateways allow SIP ALG to be disabled. It is usually a simple on/off switch or checkbox in the device’s web interface. Exactly where and how to find that setting varies from device to device, so we cannot tell you exactly how to do so in your equipment. That said, we do have some helpful guidelines and steps to follow.

How to Disable SIP ALG.

1. First, you should determine if SIP ALG is even present on your internet connection by following the guide below titled “How to Use the SIP ALG Detector”.
2. If SIP ALG is detected, open command prompt by searching for and running the program “cmd” in your windows search bar.
3. Once command prompt is open, type “ipconfig” and press enter.
4. Look for the item “default gateway”. It will be an IP address in a format similar to something like this: 192.168.1.1
5. Copy that IP address and paste it into the URL bar of your browser.
6. Login to your device with the username and password provided to you by the ISP. If you don’t know it, it might be on a sticker on the device.
7. In the user interface for the gateway settings, look for a tab titled “firewall”, “security”, or something similar.
8. Look for an item that says SIP ALG and make sure it is set to disabled.
9. Log out and reboot the gateway.

Note: If you have any issues with the steps above, can try calling your ISP customer service number and asking them to disable it for you.

How to Use the SIP ALG Detector.

1. Download and run the file that can be found at this link.
2. After installing the detector, it should automatically run (if not, you may need to run the program manually.
3. The test will be run immediately (takes only a second). It will show if SIP ALG is detected or not detected.