US patent application 2220110228919 (USPTO link) seems to be trying to patent an on/off switch for DTMF audio on telephone networks.

The key claims are these (effectively the first three):

  1. A telephone call processor for processing telephone calls comprising voice signals and data signals, the call processor comprising a first telephone interface and a second telephone interface, the call processor being operable in a first mode and in a second mode, wherein:

    • in the first mode, the call processor is adapted to receive voice signals and data signals at the first telephone interface and to transmit voice signals and data signals to the second telephone interface;
    • and in the second mode, the call processor is adapted to receive voice signals and data signals at the first telephone interface, to block data signals from being transmitted to the second telephone interface and optionally to transmit voice signals to the second telephone interface.
  2. The call processor of claim 1, wherein the data signals comprise at least one of audio tones and DTMF (dial-tone multi-frequency) audio tones.

  3. The call processor of claim 1 further adapted to switch between first and second modes in response to receiving a mode-switching data signal at the first or at the second telephone interface.

The application itself describes how this approach can restrict credit card information being sent by a customer from reaching the ears of the call center agent, or the call recording systems operating in the call center. It therefore stops call center staff from stealing card data.

The concept of stopping call center staff from accessing sensitive data transmitted by DTMF is not new. One example is Van Volkenburgh's application from 2003 Device and method for concealing customer information from a customer service representative

The concept of blocking DTMF tones is not new either, as shown by Didier Filhol's application of 1998 Telephone dialling modification accessory, e.g. to ensure use of specific service provider, has off-hook detector actuating unit to filter out DTMF signals and generate replacement signals as necessary

However, to my layman's mind, don't the claims in the 2220110228919 application above mean "sometimes DTMF tones are allowed to come through to the call center agent, and sometimes they are switched off"? In other words, it's an on/off switch.

The current application states a UK reference date of May 9, 2008.

Question: Does anybody have any prior art for this?

3 Answers 3


Here are some notes from my research.

I found a product called CallGuard which is in a similar space. The difference is that the apparatus defined in the patent application stops the real-time DTMF tones being sent to the callee's phone. Therefore the callee is never exposed to the DTMF tones where as CallGuard appears stops the callee seeing (via CTI) them and the call recorder recording them but doesn't mention the actual tones being passed to the callee's phone.

It appears the patent applicant is applying the technology to the Voip/SIP world therefore I'd expect this pattern of hiding DTMF to reside in a Session Border Controller (SBC) or the real-time configuring of the SBC if it were prior art. I wonder if there is also prior art in the PSTN world.

I agree, this application is overbroad. The special sauce looks to be detecting when PCI impacting values are entered via DTMF and removing them from the data stream but passing through general DTMF tones which dont have PCI impacts. Something a-kin what a firewall does in terms of rules/pattern matching.

The application looks to define the general problem space and not novel bits. I'd expect it to be "A method for matching DTMF tones based on rules". The real-time turning off/on DTMF is neither-nor-there and is a feature id expect from SBCs or like configurable devices.

Hope this helps.

http://www.callguard.co/callguard/how-callguard-works/ http://www.cisco.com/en/US/docs/routers/asr1000/configuration/guide/sbcu/sbc_dtmf.html#wp1168784


This is totally bogus: if you call the provider of your telephone or your bank and a computer answers your call you are almost in this situation. What could be 'innovative'is the fact that both voice recognition and button pushes (DTMF) are used in one system, but I highly doubt this...

  • 1
    I don't read it quite like that. I read it as "the voice always goes through to the agent, but the DTMF is sometimes blocked". When a computer (IVR) answers a call, the voice/DTMF isn't selectively masked/blocked. Typically, either it is all recorded, or it's not. This application seeks to selectively block DTMF from being recorded or transmitted in the call center, and allow that selection to be made during the call, and so the agent continues talking with the customer. Jul 25, 2013 at 10:14

I'm having some trouble finding an authoritative source, (paywall for specifications). http://en.wikipedia.org/wiki/Integrated_Services_Digital_Network

The opening lines of the Wikipedia page for ISDN are:

"Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book."

The Q.931 Specification (ISDN standard Protocol) from 1988 http://www.itu.int/rec/T-REC-Q.931-198811-S/en (English) Starting on Page 13 shows that the many of the 'Message Content' types may optionally use 'information describing tones' to control the Signal. This would be the same as a DTMF tone being used to control how data is sent.

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