SIP ALG (Application Layer Gateway) modifies VoIP traffic with the aim of solving NAT and firewall related problems. SIP ALG does this by inspecting SIP packets and modifying SIP Header and SDP data.
Unfortunately, SIP ALG was poorly implemented in a lot of cases, which has lead to it causing more issues than it corrects and due to this, we believe that, in general, it is best disabled.
Note - Many routers will re-enable SIP ALG after being powered off and on, or sometimes after a firmware update, therefore if it has been disabled in the past, and you know that the router was recently updated and powered off and on again, then it is always a good idea to log in to the router and double check the setting.
Virgin SuperHub: SIP ALG cannot be disabled in the settings of SuperHubs. Please click here for advice troubleshooting issues with SuperHubs.
BT: SIP ALG cannot be disabled in the settings of BT HomeHubs, but can be disable with BT Business Hub versions 3 and higher:
Fritz!Box: SIP ALG can't be disabled.
DrayTek routers: Log in to your DrayTek via Telnet using an SSH client such as Putty: http://www.putty.org/
Check if SIP ALG is Enabled or Disabled:
To check if SIP ALG is Enabled or Disabled enter this command: sys sip_alg ?
If SIP ALG is disabled a " 0 " result will be returned. If SIP ALG is enabled the result will be " 1 ".
Disabling SIP ALG:
To Disable SIP ALG enter the following:
sys sip_alg 0
The router will restart and save your changes.
Click here for additional general information about DrayTek Firewall setup.
TP-Link routers: How to Disable SIP ALG on TP-Link ADSL modem router
Linksys: Check for a 'SIP ALG' option, in the 'Administration' tab under 'Advanced'.
May also need to disable SPI Firewall.
Microtik: Disable 'SIP Helper'.
Netgear: Look for a 'SIP ALG' checkbox in the 'WAN' settings.
Port Scan and DoS Protection should also be disabled.
Disable STUN in VoIP phone's settings.
D-Link: In your router's 'Advanced' settings --> 'Application Level Gateway (ALG) Configuration' uncheck the 'SIP' option.
Huawei: Many routers support SIP ALG (usually found in the 'Security' menu).
SonicWALL Firewall: Under the VoIP tab, the option 'Enable Consistent NAT' should be enabled and 'Enable SIP Transformations' unchecked.
Test with STUN disabled in your VoIP phone's settings.
Adtran Netvanta: Disable SIP ALG under 'Firewall/ACLs' --> 'ALG Settings'.
Even if there isn't a SIP ALG option in your router's settings, it may still be implemented. TelNet commands must be used to disable SIP ALG with Technicolor, Thomson, SpeedTouch, some Draytek and some ZyXEL routers.