SIP ALG and Why It's An Issue


What is SIP ALG?

SIP ALG stands for Session Initiation Protocol Application Layer Gateway.  It is enabled by default on many commercial routers and internet gateways (modems).

This setting modifies SIP packets, which controls voice calls and faxes. 

How can it affect VoIP?

Even though SIP ALG is intended to assist users who have phones on private IP addresses, in many cases it is implemented poorly and actually causes more problems than it solves. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behavior, such as phones not registering and incoming calls failing.

Therefore if you are experiencing problems we recommend that you check your router settings and turn SIP ALG off if it is enabled.

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 Gazelle 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 your 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?

  • One-way audio when first picking up a phone call.
  • Dead air/dropped calls when first picking up a phone call.
  • Transferring calls fail.
  • Unable to put a call on hold or park on phones.
  • Unable to retrieve a call off of hold or park.
  • Inbound faxes to a fax machine failing at the start of a fax.
  • Only 1 phone in a hunt group not ringing.
  • Only 1 phone in a hunt group with multiple phones is ringing.

How do I turn off SIP ALG?

The method to disable SIP ALG will depend on your router.  You'll need to search for instructions specific to your router/gateway for instructions for your specific router or internet gateway (modem).

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.


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