• What is SIP ALG?
• How can it affect VoIP?
• Why will SIP ALG affect my phones and faxes?
• What issues will SIP ALG cause?
• How do I turn off SIP ALG?
   

What is SIP ALG?

SIP ALG (Session Initiation Protocol Application Layer Gateway) is a feature on routers that changes SIP packets. A SIP packet is a type of data packet used in VoIP (Voice over IP) to start, manage, and end voice and video calls. These packets contain details like the IP addresses of the caller and receiver, session settings, and other call-related information. SIP ALG changes these packets by altering the IP addresses and port numbers in the headers to match the external IP address and ports given by the router. This feature was designed to make sure the SIP packets reach their intended destination. SIP ALG is usually turned on by default in many commercial routers and internet modems.

How can it affect VoIP?

Even though SIP ALG is intended to assist users who have phones with private IP addresses, it often causes more problems than it solves. SIP ALG can change the packets in unexpected ways, making them unreadable. Because of these problems, we recommend turning off SIP ALG in your router settings if you are experiencing issues with your VoIP service. This can help ensure that your calls go through smoothly without interruptions.

Why will SIP ALG affect my phones and faxes?

SIP ALG modifies voice and fax packets to make them appear as if they have a public IP address and are not behind a firewall.  The setting was first designed for old VoIP phone systems that could not work behind a firewall (NAT).

Modern VoIP systems like UNIVERGE BLUE® Cloud Voice are designed to work with phones and fax adapters that are behind firewalls.  This is done to protect you from hackers hijacking your phone equipment and making fraudulent long-distance and international calls that run up your bill.

The problem with SIP ALG occurs when our servers send voice and fax data back to our network.  Your router does not know which phone/fax device to send the data back to because SIP ALG removed the private IP address of the phone from the voice/fax packets.  This makes the router unable to keep track of which phone or fax device first sent the VoIP packet.

What issues will SIP ALG cause?

SIP ALG can lead to issues like:

• One-way audio when first answering a call.
• Dead air or call drop when first answering a call.
• Call transfers fail.
• Unable to put a call on hold or to park a call.
• Unable to retrieve a call from hold or park.
• Inbound faxes to a fax machine fail at the start.
• Only one phone in a hunt group not rings.
• Only one phone in a hunt group with multiple phones rings.

This picture shows the simplified call process with SIP ALG enabled:

1. Phone data before SIP ALG is applied.
2. Phone data after SIP ALG is applied.
3. Because SIP ALG replaced the phone's Private IP with the Public IP, the returned data shows the same address for both Public IP and Private IP.
4. The modem-router does not know where to send the inbound phone data because SIP ALG removed the phones' Private IP address.

How do I turn off SIP ALG?

The method to disable SIP ALG will depend on your router.  Please search our router/gateway database for instructions for your specific router or internet gateway (modem). 

If your router or internet gateway is not listed, please refer to your manufacturer's user manual.

Alternatively, check the internet and support forums for advice.  Type your router make and model into a search engine, along with disable SIP ALG, and usually, you will be able to find out how to make this change.

SonicWall

Here's how you can disable SIP ALG on SonicWall:

1. Navigate to Network > VoIP > Settings 
2. Enable Consistent NAT 
3. Disable the option Enable SIP Transformations
4. Press Accept