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 a 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.
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.
Most routers include a firewall, which is a filter that blocks traffic that should not be allowed in or out. For smaller offices with off-the-shelf routers, these firewalls may need to be modified or turned off. On enterprise-level equipment, certain rules may need to be added. The IT administrator should make these adjustments.
The following settings listed below can typically be found in a commercially available router’s web interface and can be easily checked on/off…
It is recommended to have the customer’s IT administrator apply the following changes listed below to ensure it is done properly…
To ensure that traffic is not being blocked, the following ports used for VoIP communication to our servers should be opened:
• SIP Ports 5060, 6060
• RTP Ports 10000 – 49999
Other Router/Firewall Settings
• Make sure our IP is set as a trusted source: 220.127.116.11/23
• Increase UDP timeout settings up to 120
• Disable SIP Transformations
• Enable consistent NA
SPI (Stateful Packet Inspection) / DoS Protection
SPI/DoS allows the router to approve or deny any information packets for security reasons. Oftentimes it will incorrectly identify our VoIP traffic as a security risk. DoS protection keeps track of how many connections are made to an individual web address and begins blocking access once a limit is reached. Our phones connect to the same site. The more phones in your network, the more likely SPI/DoS will begin to block connections. To prevent this, some routers will allow you to either disable SPI/DoS protection to allow more connections.