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.
SIP ALG can not be disabled in the settings of SuperHubs. Please click here for advice troubleshooting issues with SuperHubs.
SIP ALG can not be disabled in the settings of BT HomeHubs, but can be disabled with BT Business Hub versions 3 and higher.
SIP ALG can not be disabled.
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.
Check for a SIP ALG option in the NAT or Firewall settings if there's a SIP ALG 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 username and/or password. These are the same as you use 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.
Check for a 'SIP ALG' option, in the 'Administration' tab under 'Advanced'.
May also need to disable SPI Firewall.
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.
In 'Advanced' settings --> 'Application Level Gateway (ALG) Configuration' un-tick the 'SIP' option.
The SIP ALG setting is usually found in the 'Security' menu.
Under the VoIP tab, the option 'Enable Consistent NAT' should be enabled and 'Enable SIP Transformations' unchecked.
Under 'Network' or 'Advanced' --> 'ALG' un-tick the options 'Enable SIP ALG' and 'Enable SIP Transformations'.
Disable SIP ALG under 'Firewall/ACLs' --> 'ALG Settings'.
TelNet commands must be used to disable SIP ALG with some Technicolor, Thomson, Speedtouch, DrayTek and Zyxel routers.