question: Why dont speakers make the characteristic intereference sound anymore when there is an incoming call?
comment: Ha yes, I remember that. You used to be able to hear that even before the phone rang. The reason why it happened is a bit too complicated for me, but the reason why it doesn't happen anymore is that we don't (or barely) use 2G (GSM) networks anymore. These networks were on the 800-900 MHZ frequencies.
passages: People using telecommunications devices for the deaf (TDDs) and calling public agencies might expect the agency being called to answer with a TTY. The caller waits for "TTY tones" to be generated over the phone line. The answerer should place the phone receiver into the TTY coupler and type anything ("Hello, Go Ahead") so the caller knows it is connected. For public agencies that have a TTY, it is inappropriate to hang up on a silent call.
   These acoustic technologies must also remove or reduce the noise levels so that the caller is well understood. A person making a call from a handsfree device who is in a busy restaurant or while driving will introduce large levels of noise into the call. This situation is complicated as the software must not only remove the noise around him, but must transmit his voice clearly and loudly to whoever is connected to him. 
   Ambient noise levels can impede both collections of the initial and subsequent voice samples. Noise reduction algorithms can be employed to improve accuracy, but incorrect application can have the opposite effect. Performance degradation can result from changes in behavioural attributes of the voice and from enrollment using one telephone and verification on another telephone. Integration with two-factor authentication products is expected to increase. Voice changes due to ageing may impact system performance over time. Some systems adapt the speaker models after each successful verification to capture such long-term changes in the voice, though there is debate regarding the overall security impact imposed by automated adaptation.
   Other countries have sought to use technical means to prevent callback, by blocking the inbound and outbound calls made to and from DID numbers, or by disabling touch tone dialing. However, this is also overcome by the use of speed dial numbers, while others use these services by triggering callback via the Internet or SMS.
   Silent calls, calls which remain silent when answered, are often due to callers using an automated dialer to make multiple calls in rapid succession, anticipating that many will not be answered; if an operator is not available at the time a call is answered, it will be silent. Silent calls, while not covered by TPS, can be reported to Ofcom.
  
  Section::::Unwanted calls not covered by TPS.:Automatic messages.
   When silence suppression is active, the line appears to have gone dead at the other (egress) end of the call. For this reason, so-called "comfort noise" needs to be generated to compensate for the lack of background noise. The ingress end must therefore signal the egress end that silence suppression is in effect. For best results, the level of comfort noise being generated on egress should match that of the background noise at the ingress end.
  
  Speech activity detection must occur very quickly, otherwise clipping might occur.
   Section::::The No Agent Available (NAA) message.
  
  One solution to silent calls is for marketing organizations to play a recorded message to the consumer explaining why the call was silent and why no agent was available. The industry, led by the DMA, resisted this for a long time because it was assumed that it would be termed illegal under article 19 of the Privacy and Electronic Communications (EC Directive) Regulations 2003 (PERC):
  
  "Use of automated calling systems
   The use of an electrolarynx can cause social issues, for instance difficulty ordering food, drinks, or other items in noisy environments; or, when answering a telephone, having the caller respond, "Am I talking to a computer?"
  
  However, quality-of-life improvements due to electrolarynx usage are generally significant. One user states: 
   Many silent calls are a result of process known as pinging. This is very similar to an Internet Protocol (IP) ping, where the intention is to see if there is life at the destination. This is often used by data cleaning companies as a method of removing dead telephone numbers from old lists. Each number is dialed and immediately dropped if the call is connected. If the call is not dropped quickly enough, it may result in a short ringing signal.
  
  Section::::Telemarketing and the UK DMA.
   BULLET::::- the caller cannot set a Ring Back if urgent contact with the called party is required;
  
  BULLET::::- many answering machines implement call screening, which 1571 cannot do: the answering machine picks up the call with its speaker active, so that the user, if available, can listen to what the caller says, and pick up the phone only if they want to speak with the caller;
  
  BULLET::::- some machines can record conversations; 1571 cannot.
   All fully selective ringing systems on party lines still brought the inconvenience of finding the line in use occasionally, by hearing talking, when one picked up the phone to make a call. All party lines also required special equipment to complete calls to another party on the same party line.
  
  Section::::Characteristics.
   The challenge with VoIP on mobile devices is to make sure that the device is ready to receive incoming calls while keeping the power consumption as low as possible. Due to the inherent mobility of mobile devices, the network conditions change often and frequent SIP re-registrations and keep-alive traffic are needed to make sure the mobile client is properly registered and will receive incoming call at all times. This has a significant impact on battery life.
   There are strong concerns that the ever-listening microphone of smart speakers presents a perfect candidate for a wiretap. In 2017, British security researcher Mark Barnes showed that pre-2017 Echos have exposed pins that allow for a compromised OS to be booted.
  
  Section::::Privacy concerns.:Voice assistance vs privacy.
   With the widespread phase-out of the ACTS signaling system, and indeed the long-completed phase-out of MF signaling in North America, the telephones for which this device was designed are no longer in service. Thus the green box can be considered obsolete.
   As call centers began to migrate to multimedia in the late 1990s, companies started to invest in computer telephony integration (CTI) with IVR systems. IVR became vital for call centers deploying universal queuing and routing solutions and acted as an agent which collected customer data to enable intelligent routing decisions. With improvements in technology, systems could use speaker-independent voice recognition of a limited vocabulary instead of requiring the person to use DTMF signaling.
   In Europe, call waiting was also introduced in the 1970s with the introduction of the first digital switching systems such as the Ericsson AXE, Alcatel E10 and System 12. It was also available on some Ericsson crossbar exchanges, such as the ARE11 which, while electromechanical, was also computerised. Other digital systems such as the UK's System X also supported the service. Switching systems developed in the 1980s such as the German Siemens/Bosch EWSD also had call waiting.
  
  Section::::See also.
  
  BULLET::::- Internet Call Waiting
  
  BULLET::::- Vertical service code
  
  Section::::External links.
   With the application of newer equipment, music on hold devices can now interact with the caller. No additional programs are required on the platform, as all the logic is done with the MoH device. This can include services such as 'polling on hold', rating your customer service officer anonymously, etc. 
  
  The trend toward hosted IP telephony for business phone systems is demanding changes in music on hold message technology.
  
  Section::::Legal aspects.
  
  Section::::Legal aspects.:Copyright law.
   Radios with DTMF decoders may monitor all system traffic or remain muted until called, depending on the system design. When the radio receives the correct digit string, it may momentarily buzz or sound a Sonalert. An indicator light may turn on and remain latched on. In most systems, the radio's receive audio would latch on after receiving a valid digit string if normally muted.
   On most European telephone networks, and GSM or UMTS mobile phones, call waiting is activated by dialing the following codes:
  
  BULLET::::- To activate: *512*
  
  BULLET::::- To deactivate: #511#
  
  BULLET::::- To check status: *1645#
  
  A voice announcement, tone or a message on your phone's screen will confirm the service status.
   BULLET::::- Due to the latency induced by protocol overhead and other factors they do not work as well on satellite Internet, analog cell ("edge" networks) and other high-latency Internet connections. Extremely latency sensitive applications (music, remote device control) as of 2012 simply cannot exploit VoIP protocols.
   In packet radio systems such as GPRS/EDGE, it is possible to combine DTX with capacity increase when VoIP is used for telephony. In such cases, resources freed up when one user is in silence can be used to serve another user. The increase of the number of users will contribute to the interference level. Systems that use voice codecs such as AMR can reduce vocoder rate adaptively to better combat interference. 
   With the release of iOS 6, which was released on September 19, 2012, Apple added features that enable the user to have options to decline a phone call when a person is calling them. The user can reply with a message, or set a reminder to call them back at a later time.
  
  Section::::Software.:Multimedia.
   A CNN survey found that 70% of callers in the United States who are holding the line in silence hang up within 60 seconds. A study by North American Telecom found that callers hearing music on hold will stay on the line 30 seconds longer than callers experiencing silence, and callers hearing commercials on hold will stay on the line for up to 3 minutes longer.
  
  In November 2011 an omnibus survey of more than 2,000 UK consumers highlighted several key figures related to modern business
   In the U.S., the Federal Trade Commission (FTC) uses the term "abandoned call" instead of silent call in its regulations applying to telemarketing. "Abandoned call" in non-FTC contexts may refer to a caller who decides not to await answer before hanging up.
  
  For more information on why the technology makes silent calls, see predictive dialer.
  
  Section::::Other reasons for silent calls.
   BULLET::::- "Type I" is the most common standard in North America and Japan, and signals an outgoing call from the PBX to the signaling unit with Battery on the M ("Mouth") lead. Incoming calls from the CO are signaled by an earth (ground) signal on the E ("Ear") lead. The interface is vulnerable to poor earthing at either end, and interference from external electrical noise which act on inferior voice signal quality.
  