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.
SIP ALG is often poorly implemented leading to many issues and is, in general, best disabled.
Many routers re-enable SIP ALG after being powered off and on.
After disabling SIP ALG please test with and without STUN enabled in your phone settings.
Disable SIP ALG under Firewall/ACLs -> ALG Settings.
Disable the option SIP Passthrough under Advanced Settings / WAN -> NAT Passthrough.
If your router doesn't have this option SIP ALG may be disabled via Telnet.
SIP ALG can not be disabled.
Check for a SIP ALG option in the NAT or Firewall settings.
SIP ALG can not be disabled in the settings of BT HomeHubs, but can be disabled with BT Business Hub versions 3 and higher.
In 'Advanced' settings --> 'Application Level Gateway (ALG) Configuration' un-tick the 'SIP' option.
With Vigor2760 devices the option can be found in the regular interface at Network -> NAT -> ALG.
With all other devices you'll need a telnet client.
On Windows systems press the Windows Start button, search for cmd and hit enter.
Type in telnet 192.168.1.1 and hit enter again.
If you don't have the Windows telnet client installed, please go to Start -> Control Panel -> Programs -> Programs and Features -> Turn Windows Features on or off and ensure Telnet Client is checked and click OK.
You will be prompted to provide a username and/or password. These are the same credentials used to access the router's web interface.
Afterwards, type in these commands:
sys sip_alg 0
On Draytek Vigor2750 and Vigor2130 please use these commands instead:
kmodule_ctl nf_nat_sip disable
kmodule_ctl nf_conntrack_sip disable
Click here for additional general information about DrayTek Firewall setup.
The SIP ALG setting is usually found in the Security menu.
Check for a SIP ALG option in the Administration tab under Advanced.
May also need to disable the SPI Firewall option.
Disable SIP Helper.
Look for a 'SIP ALG' checkbox in 'WAN' settings.
Port Scan and DoS Protection should also be disabled.
Disable STUN in VoIP phone's settings.
Under the VoIP tab, the option 'Enable Consistent NAT' should be enabled and 'Enable SIP Transformations' unchecked.
Telnet commands must be used to disable SIP ALG with some Speedtouch routers. Please refer to the manufacturers support documentation.
In the Features menu -> Advanced -> NAT ALG Status please disable the SIP entry.
Should you use port forwarding on this device, please also disable the UPnP option on the same page.
Telnet commands must be used to disable SIP ALG with some other Technicolor routers. Please refer to the manufacturers support documentation.
SIP ALG can not be disabled in the settings of SuperHubs. Please click here for advice on troubleshooting issues with SuperHub devices.
Under Network or Advanced -> ALG un-tick the options Enable SIP ALG and Enable SIP Transformations.
Telnet commands must be used to disable SIP ALG with some other Zyxel routers. Please refer to the manufacturers support documentation.