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A Love-Hate Relationship With SIP

The technology is convenient but comes with a major nuisance: robocalls

The author is director of sales at Comrex.

SIP-based VoIP telephony can be considered a technological blessing due to its efficient implementation, cost savings and audio quality.

Many of us utilize low-cost VoIP phone lines in our homes and businesses every day and save lots of money as a result. In addition, there is an over-abundance of free and low-cost SIP apps available for mobile devices that bring this technology into our smartphones, allowing us to make free phone calls using our cellular data plan or a Wi-Fi hotspot.


For broadcasters, these apps have become great tools for news contribution and short-form outside broadcasts. The added benefit of high-quality voice algorithms like G.722, AAC-ELD and Opus allow these SIP apps to send studio quality audio from the field to hardware audio codecs that support the EBU’s SIP-based Tech3326 Interoperability standard. It’s nice to have these extra “tools in the toolbox” to supplement a station’s arsenal of professional hardware audio codecs.

But as with all good things that innovation brings to the world, someone finds a way to exploit the technology for more nefarious purposes.

If you are one of the millions of subscribers of VoIP telephone service, you may have noticed an uptick in the frequency of annoying robocalls delivered to your home, office or even cellphone. These usually happen as you are sitting down to dinner or as you are just getting your children to sleep.

Offers to lower your credit card interest rate or get a free vacation can be infuriating; but this type of invasive marketing relies on volume of calls to produce results.

VoIP- and SIP-based calls typically use the public internet and the common UDP port 5060 to send and receive the transmitted audio. So if you have VoIP service on your network, UDP port 5060 generally is open on the router or firewall connected to the Internet. SIP Bots constantly scan the internet looking for this open port and, once they find it, will start sending the annoying robocalls to your IP address and, in many cases, “spoofing” their caller ID to make it look like a call from a neighbor or legitimate location. For broadcasters, this can be even more frustrating.

Since EBU Tech3326 is SIP-based, UDP port 5060 commonly is open to the public internet on professional hardware audio codecs. Not only does this allow for these codecs to receive calls from other hardware codecs (including those from different manufacturers) but also from the previously mentioned SIP apps. Just as the number of robocalls has recently been on the increase, so too has the instance of unsolicited traffic to professional hardware audio codecs. This typically will manifest itself in the hardware codec appearing to be busy or “locked up” when in fact, a SIP Bot auto-dialer has connected to the device.


Because hardware codecs aren’t typical VoIP devices, SIP signaling and functionality can be slightly different and a disconnect command might not be issued resulting in a “busy” unit. In 2014, Comrex added a SIP Bot “black-listing” feature to the firmware for our IP audio codecs. This feature identifies common SIP Bots and will prevent them from connecting to our hardware codecs such as ACCESS, BRIC-Link and BRIC-Link II. However, we often advise customers simply to disable the “Accept Incoming SIP Calls” feature when not in use to make it less attractive to SIP Bots and their mutating, artificially intelligent offspring.

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Thanks to SIP technology, broadcasters enjoy the freedom to create unique content from more locations than ever before. While SIP can certainly be a blessing, it can be a curse as well. In order to achieve the maximum benefit from any technology solution, making sure that you have a plan for best practices in place is critical. Also, consulting with an IT professional that understands the intricacies of broadcast can save a multitude of headaches down the road.