If you are experiencing one way voice particularly on incoming calls the most probable cause is SIP ALG. While originally designed to resolve a NAT related problems, with no standard implementation many routers and software based firewalls corrupt the SIP message by attempting to rewrite part of the SIP message. Frustratingly the one-way voice issues will often only occur on a single number and will clear once the router has been reset.
As a general rule all VoIP service providers recommend disabling SIP ALG (see VoIP Org).
1. Disabling SIP ALG – Cisco IOS Router
- From the CLI: Enable configure terminal, No IP nat service SIP UDP port 5060
- For TCP run: No ip nat service sip tcp port 5060
2. Disabling ALG – DrayTek Routers
This guide specifically applies to the Vigor 2760 but is also applicable to most other DrayTeks in the series.
- 2800, 2820, 2830, 2860, 2920, 2925, 2960
- Start | Cmd
- Type: telnet 192.168.1.1 (Enter) NB – Ensure Telnet is enabled ( Start | Control Panel | Programs | Programs and Features | Turn Windows Features on or off | TELNET CLIENT >> OK)
- Default user name: admin | admin
- Type: sys sip_alg 0 (Enter) Enter
3. Disabling ALG – Juniper JunOS Router
- From the CLI: To verify if SIP ALG is enabled or disabled run show security ALG status | match sip
- To disable run:
- Set security alg sip disable
4. Disabling ALG – Netgear Modem Router
This guide applies to most Netgear routers
- In browser browse to 192.168.0.1 (this is the default IP address)The default username is admin and the default password is password
- Under Advanced select WAN Setup
- Check Disable SIP ALG option
- Select Apply
5. Disabling ALG – SonicWALL
This KB applies to all SonicWALL SonicOS firewalls.
SonicWall has a feature called SIP Transformations that consistently cause total or partial loss of voice.
- Log into the web interface on the SonicWall.
- On the left, find the VOIP tab. Depending on the version of SonicOS your screen may appear slightly different.
- Disable SIP Transformations: Browse to the SIP Settings page and ensure Enable SIP Transformations is NOT enabled.
- Enable Consistent NAT: The Consistent NAT setting will ensure the same NAT port is used. If this is disabled, you may experience undesired call behaviour.
- Ensure Consistent NAT is Enabled
6. Disabling ALG – TP-LINK Modem Router
This KB applies to most TP-LINK routers.
On Windows 7/8
- Start | cmd >>Enter
- Type:telnet 192.168.1.1EnterNB – Ensure Telnet is enabled ( Start | Control Panel | Programs | Programs and Features | Turn Windows Features on or off | TELNET CLIENT >> OK)
- Default user name: admin | adminNote that no symbols may appear when typing in the password, but continue anyway.
- Type inip nat service sip sw offEnterIf successful, it should say Nat SIP v2 switch off!
7. Disabling ALG – Zyxel Equipment
Web UI: Using the Web Interface Navigate to the NAT tab and uncheck the box for SIP ALG.
- Telnet into the router Select menu items 24 then 8
- To display current SIP ALG status run the following command:
8. Disabling ALG – Netcomm
While the following KB guide specifies the NB604N all Netcomm routers are similarly configured. Netcomm also provide a useful router emulator for all their routers.
- Logon to the router
- Advanced Setup | NAT | ALG >> uncheck “Enable SIP”