Although VoIP devices are designed to work with all types of routers, you may encounter issues getting VoIP to work properly, and some of your network settings or hardware may need to be modified.
Here are some general observations and rules, based on our experience. Consult the user guide for your network component or contact the manufacturer’s support department if you need specific guidance.
Make Sure You Don’t Have A “Double NAT” Network
“Double NAT” (network address translation) is an situation where two routers are placed one after another. This can be a common scenario if, for example, your Internet Service Provider (ISP) provides a modem/router combination device, but you’ve also added a wireless router of your own. Although a computer behind a double NAT setup will usually work fairly well, a VoIP phone will exhibit unusual behavior.
To resolve this, you’ll need to put your modem/router device into “bridged” mode—most models will allow this. Doing so will disable all routing (NAT) functions on the modem/router, so that only your standalone router will perform these functions. Contact your ISP if you need specific guidance.
You could also choose to purchase a standalone modem, to avoid dealing with your ISP’s hardware all together.
Disable the SIP Application Layer Gateway (ALG)
Originally designed to be a security component to help certain types of traffic get through your router, most SIP ALGs actually make things worse.
Not all routers have SIP ALG enabled if they even have it to begin with, but those that do will interfere with VoIP traffic, often preventing traffic from reaching your phone correctly or at all. We’ve seen all of the following behaviors:
- Your phone might not get dial tone
- You could drop out in the middle of a call
- Your phone might not always ring on incoming calls
- The wrong phone might ring (if you have more than one)
- Your phone might continue to ring after being answered
If you have a router that doesn’t let you disable SIP ALG, consider purchasing a different router that either doesn’t have SIP ALG, or lets you disable it. Take a look at our Network Hardware Recommendations for general guidelines.
If replacing your router isn’t a viable option, you could put one phone in your router’s DMZ. Although this will expose your phone to the open Internet, it eliminates the SIP ALG issue without requiring you to purchase new hardware.