Submitted by Linda W Reichert

The Special Information Tone (SIT) for vacant or blank numbers can and does stop robo-callers. SIT tones are three precise, sequential tones which are used by telephone companies to convey information about the condition of the phone line. The most commonly used SIT tone is the vacant number intercept SIT, that boo-eee-ooo sound you get when a number is no longer in service.

For landlines, technology already exists that allows an individual user to produce the vacant number intercept tone at home. These devices can be simple black boxes hooked up between the phone jack and the phone (see exhibit 1-“tmstopper.pdf”), while others include a handset, which I use (see exhibit 2-“vetch telezapper manual”). These devices are intended to work against robo-callers by causing the robo-callers “predictive dialing” software to drop the target number off the call list, thereby reducing and eventually stopping all robo telemarketing calls.

Currently, disconnected cell phone number are not announced with the vacant number intercept SIT and no apps exists for cell phones that allow the user to produce the vacant number intercept SIT, mainly because it is not needed.


Using the vacant number intercept SIT on landlines is simple, cheap and most importantly, effective. Through personal data collection on my home number, I went from 5 robo-calls per day on Oct 29th, 2012 to 0 robo-calls on Election Day, Nov. 6th 2012.

Let me repeat this. I received ZERO robo-calls on Election Day 2012 after using a Tele-Zapper phone for 8 days prior to November 6, 2012. While one of these blocked calls might have come from someone I was willing to listen to, most calls I received before hooking up my Tele-zapper were from PAC’s advocating for or against propositions on the ballot here in California. And if any calls were from a political party, I surely didn’t miss them. After the SIT went out, I could answer the call by then saying hello and if the call was from a real person (therefore not a Robo-call) I could listen to their spiel (if I so chose).

This will fool predictive software that uses the time it takes for “initial voice activity” to determine if a person answers as opposed to an answering machine and definitely will fool software listening for the SIT for a vacant number.

To reproduce this for cell phones, wireless carriers need to issue a standardized tone for invalid cell phone numbers that can then be re-produced by the subscriber. As with landlines, this will cause the prediction dialing software used by robo-callers to drop their number off the call list.

This first step, as it applies to landlines, has already been done. The second step-getting wireless carriers to produce a standardize tone for disconnected cell phone numbers and enabling users to produce the tone themselves- will be quick and easy to implement as it can all be handled by software by the wireless carrier; for the user, a simple app will do; no new phones or devices required.


Using a Tele-zapper or any device on a phone line to re-produce the disconnected number SIT could not be easier. Anyone can plug in the device between their current phone and the wall plug. For cell phone lines, as I stated, a simple app that can be downloaded by users will produce the same effect for wireless lines.


Already done for land lines. For cell phones, it may require regulation to require wireless providers to produce a comparable tone for all disconnected cell phone numbers. As stated before, this can be accomplished through software and will not require new cell phone to be purchased by users.

One important aspect of this solution is that however the phone companies and wireless providers produce the SIT, it must be exactly re-producible by users.

For example, on land lines when it’s the user sending out the SIT, first an off-hook signal is sent to the local switch when they answer the phone (-48 volts) and then the SIT is heard. This must be the same when it is the phone company issuing the SIT for a truly disconnected number. Otherwise, software can be written to find any differences between users and the phone company. The same caveat applies to wireless providers.

In summary, this approach works, is easy to implement and hard to defeat. And from personal experience, IT WORKS.

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