What Is SIP ALG and Why You Should Disable It

What Is SIP ALG?

SIP ALG (Application Layer Gateway) is a feature built into most business and consumer routers that is designed to help VoIP traffic pass through NAT (Network Address Translation). In theory, it inspects and modifies SIP packets to ensure they are routed correctly. In practice, it is the single most common cause of VoIP problems on business networks.

SIP ALG is enabled by default on the vast majority of routers sold in the UK, including those supplied by BT, Sky, Virgin Media and most third-party manufacturers. If you are experiencing VoIP issues, disabling SIP ALG should be your first troubleshooting step.

Why Does SIP ALG Cause VoIP Problems?

SIP ALG was originally designed for a good reason — to help SIP traffic traverse NAT firewalls. However, the implementation on most routers is fundamentally flawed. Here is what goes wrong:

It Rewrites SIP Headers

SIP ALG intercepts SIP packets and rewrites the IP addresses and port numbers in the SIP headers. Modern VoIP systems and SIP providers already handle NAT traversal themselves using techniques like STUN and keep-alive packets. When SIP ALG also modifies the packets, it creates conflicting information that breaks the call setup process.

Common Problems Caused by SIP ALG

One-way audio — you can hear the caller but they cannot hear you, or vice versa (see our one-way audio troubleshooting guide)
Calls dropping after 30 seconds — the modified SIP headers cause the call to fail after the initial setup
Registration failures — phones show "No Service" or fail to register with the SIP server
Incoming calls not ringing — the rewritten contact headers prevent inbound calls from reaching your phones
Intermittent call failures — calls work sometimes but fail unpredictably
Echo or poor audio quality — caused by packet manipulation and increased latency

How to Check If SIP ALG Is Enabled

Most routers do not make it obvious that SIP ALG is active. You will typically need to:

Log into your router's admin interface (usually at 192.168.1.1 or 192.168.0.1)
Look in the Advanced, Security, Firewall or NAT settings sections
Search for settings labelled SIP ALG, SIP Passthrough, SIP Transformations or ALG

If you cannot find the setting, consult your router's documentation or contact your ISP.

How to Disable SIP ALG on Common UK Routers

General Steps

Open a web browser and navigate to your router's admin page (typically 192.168.1.1 or 192.168.0.1)
Log in with your admin credentials (check the sticker on your router for defaults)
Navigate to the Advanced Settings, Firewall or NAT section
Find the SIP ALG or SIP Passthrough option
Set it to Disabled or Off
Save the settings and reboot the router

BT Smart Hub / Smart Hub 2

Log into the BT Smart Hub at 192.168.1.254. Navigate to Advanced Settings > Firewall. Look for SIP ALG and set it to Off. Save and reboot. Note: some BT Smart Hub firmware versions do not expose this setting, in which case you may need to use a third-party router.

Sky Router (Sky Hub / Sky Q Hub)

Sky routers have limited configuration options. Access the admin page at 192.168.0.1 and check under Advanced > Security. If SIP ALG cannot be disabled, consider placing a third-party router behind the Sky hub in bridge mode, or using a dedicated VoIP-friendly router.

Virgin Media Hub

Log into the Virgin Hub at 192.168.0.1. Navigate to Advanced Settings > Firewall. Disable SIP ALG if the option is available. On some Virgin Hub models, you may need to put the hub into Modem Mode and use your own router that allows SIP ALG to be disabled.

Netgear Routers

Access the admin page at 192.168.1.1 or routerlogin.net. Go to Advanced > WAN Setup. Uncheck Disable SIP ALG (note: on Netgear routers the checkbox wording can be confusing — ensure ALG is actually turned off). Save and reboot.

TP-Link Routers

Log in at 192.168.0.1 or tplinkwifi.net. Navigate to Advanced > NAT Forwarding > ALG. Uncheck SIP ALG. Save and reboot the router.

Testing After Disabling SIP ALG

After disabling SIP ALG and rebooting your router:

Reboot all VoIP phones — they need to re-register with fresh SIP sessions
Make a test call — call a mobile and verify two-way audio works
Test incoming calls — call your number from a mobile to confirm inbound calls ring correctly
Check registration status — verify all phones show as registered in your phone system admin panel
Monitor for 24 hours — some SIP ALG issues are intermittent, so monitor call quality over a full day

What If You Cannot Disable SIP ALG?

Some ISP-supplied routers do not allow SIP ALG to be disabled. In this case, your options are:

Use your own router — purchase a VoIP-friendly router (such as a Draytek or UniFi) that allows full control over SIP ALG settings
Bridge mode — put your ISP router into modem/bridge mode and connect your own router behind it
Contact your ISP — some ISPs can disable SIP ALG remotely on their managed routers

If you are still experiencing VoIP issues after disabling SIP ALG, check our VoIP troubleshooting guides for further assistance, or contact our team for expert support.