SIP ALG modifies VoIP traffic (packets) and is enabled by default on many routers. When implemented correctly, SIP ALG modifies specific IP addresses in these packets, changing them from your local IP address to your public IP address. Unfortunately, in many routers it is poorly implemented and, instead of replacing only the specific IP addresses that need to be replaced, it replaces all of them, which then breaks SIP signalling. This poor implementation of SIP ALG can cause several issues, such as:
- Calls dropping after a set period of time
- Calls dropping when trying to retrieve the call from hold
- Calls dropping when being transferred
- Calls not being received
- Unable to make outbound calls
- Calls with no audio
- Calls with one way audio
- Other extensions continue ringing after call has been answered
Not all routers offer the option to disable SIP ALG.
Best Solution: Disable SIP ALG On Router
Some routers offer the option to disable SIP ALG. This is usually done by navigating to the router's IP address, entering the username and password (usually 'admin') on the back of your router, and finding and disabling the SIP ALG option. A few router-specific guides can be found by clicking your router make in the list above.
Alternative Solution: Change VoIP Phone Ports
If your router does not allow you to disable SIP ALG, or you do not have access to your router, the solution is to change the port your VoIP phone uses to 5065. By default the port will be set to 5060; a standard SIP port. By changing this port, it is likely that packets sent from your phone will not be flagged as SIP packets by your router, and so will not be affected by SIP ALG. To change this port, connect to the phone by entering the phone's IP address into the address bar of a web browser on a computer on the same network as the phone. Specific instructions for various models of phone can be found on the Handset Configuration page.
Once you have connected to the phone, you will need to change the following settings on each account that you use on the phone:
- Sip Server: sip.sipcentric.com:5065
- Local SIP Port: 5065
- Proxy Server Port (If using a proxy): 5065
NB: If you are using multiple accounts on the same phone, ensure that you don't use the same Local SIP Port on multiple accounts. Each account must have a unique Local SIP Port. We recommend using 5065 for the first, 5066 for the second, and so forth.