SIP ALG is a feature that sometimes prevents our traffic from flowing properly. SIP ALG can cause various issues, such as a loss of connection with the service, calls disconnecting, or no audio from the beginning of a call. Depending on the device, it is sometimes recommended that you disable these features.

On Cisco routers, this is usually referred to as SIP inspection and can be disabled with the command “no inspect sip” or “no fixup protocol sip 5060.”

Network equipment is often customized for VoIP, but some of these customizations can interfere with our traffic. Our system does not require specific VoIP-supporting functions, which goes hand-in-hand with SIP ALG.

SPI/DoS protection allows the router to approve or deny any information packets for security reasons; however, it often incorrectly identifies 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 all connect to the same site. While a couple of phones may not cause a problem, the more phones there are, the more likely SPI/DoS protection will begin to block connections. To prevent this, some routers will allow you to either disable SPI/DoS protection or allow more connections.

These protocols are used to send and receive traffic over the Vonage network. If you are using a firewall to secure any of these protocols, it may inhibit your ability to make or receive calls. Usually, these phones cannot have any outbound restrictions in place within your firewall. For more advanced equipment, if your firewall allows you to edit your UDP session timeout, ensure that it is set to 30 seconds or more.

RTP (Voice) Traffic: Audio packets are sent using RTP random ports between 20000 and 30000. If any of these ports are blocked, you can experience one-way or no way audio. 

Ideally, you only need one device to perform routing functions. Double-NATing (double-routing) is known to cause problems for VoIP phones. It is best to eliminate or bridge any extra routers or modem/router combinations on your network. If you need to put your modem/router combination in bridge mode, please contact your internet service provider (ISP) for assistance. If bridging is not possible, you may also try putting the second router in your first router’s DMZ; however, this method is not guaranteed to work.