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Disable SIP ALG On Your Routers


Some broadband routers have a SIP ALG enabled. Often this can cause issues with VoIP calls. It is important that you disable SIP ALG where possible

SIP Application Layer Gateway (ALG) is enabled on the majority of common enterprise, business and home broadband routers. It's purpose is to inspect VoIP traffic packets and prevent problems with the firewall by modifying the packets to permit connections to the protocols and ports required

VoIP phone systems require SIP ALG to be turned off as it causes a variety of problems as it adjusts and terminates VoIP traffic packets and causes them to become corrupted and this causes issues with VoIP, such as: one way audio, dropped calls, problems transferring calls, handsets losing registration and making or receiving internal calls

For information on how to turn off the SIP ALG on your router please refer to your routers user guide or you can have a look at some information about known router issues with SIP ALG on some common routers we have tested

UDP Fragmentation

In some cases problems with VoIP services can be caused by UDP fragmentation, for example the size of the UDP datagram packets sent between VoIP phone systems and your phones will be greater than the default 1500 bytes, this will cause UDP fragmentation. In order to prevent problems with UDP fragmentation you need to ensure that all network equipment you are using with your VoIP phones support UDP fragmentation. If UDP fragmentation is not supported then a number of problems can occur, such as: issues with BLF (Busy Lamp Field) and Feature Synchronisation

Known Router Issues

Known Compatible Routers

See this spreadsheet

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