PBX Voice CAPTCHA And Why It’s Amazing

Generally speaking, one of the most undervalued servers I host is an Asterisk PBX build.  This VoIP server is home to roughly 15 active extensions, 10 DID phone numbers, and three conference lines spaced across several locations. On average the server processes 80-100 inbound and about 10 outbound calls per month.  The server used to process at least two or three times that amount per month until internet messaging and other dependencies changed the demand.

The few publicly advertised DID numbers now accept roughly one or two spam robo calls per hour.  Similar to spam e-mail processing, there is currently no way to ensure 100% accuracy in filtering based on defined rules or metrics.  This is where I had to think creatively about what makes Internet browser based CAPTCHA devices so successful and how to incorporate that level of accuracy into a PBX system.


The solution was easier than I initially thought. My first attempt involved adding a simple Interactive Voice Recording/Response (IVR) to the phone number’s inbound route.  The IVR message itself was a quick recording that says, “Hello! I really hate spam robo calls, so if you’re a human, go ahead and press five and you’ll be connected to me. Thanks!” 

The recording was processed using audio properties that make it slightly harder for speech to text translators to grab what I’m prompting.  This was done to extend the usefulness of this process as artificial intelligence evolves in VoIP.

Amazingly enough, this simple process is still in place and does not need improvement or any additional filters to be applied. The IVR has been in place for over six months and has a 100% effective rate.

Some factors I have considered that may need to be expanded on if this is something to be replicated elsewhere:

  • The message itself sounds like the beginning of a voicemail greeting.  This may deter potential clients or prospects calling into a commercial line.  Generally though people have come to expect an IVR greeting when calling a business, however it may be worth a discussion.
  • If artificial intelligence becomes advanced enough to detect the directions to simply press a number or two in order to get through, this may require updating the model to include an intelligence test such as a simple math equation or well known fact as an answer. Similar to Internet browser based Captcha tests, these take longer and increase client frustration so it’s again worth a discussion.