What is SIP ALG?
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
Related Articles
[DOWNLOAD] OfficeLine
Make and receive calls on your mobile device or computer using nexogy's softphone application. Auto-attendants, ring groups, and queues will behave exactly as if your entire team was in the office. Start using it NOW with the Web Browser version or ...VLAN Configuration
VLAN configurations vary depending on the phone Brand however they all function the same way, the packets need to be tagged and then you can route the packets tagged to the connection you want to use for voice. But the phones would need to be able to ...