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.
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).
- 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 call 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.
Please refer to the article on how to disable this configuration: Firewall Rules