• Customer calling features
  • Custom local area signaling service (CLASS) features
  • Voice mail

Customer Calling Features – A common set of custom calling features are available such as: call forwarding, call waiting, three way calling (for conferencing), speed dialing (storing frequently used numbers), added number (add second line).

Class Features – Class features give subscribers control over incoming/outgoing calls.  This standard is defined by Bellcore (Bell Communication Research).  The lists of features that come under Class are described below:

  • Customer oriented trace – enable subscriber to dial a code on receiving a harassment call to send notification to local law enforcement agency.
  • Automatic callback – This feature intimates subscriber when line is free.
  • Automatic recall – enable subscriber to return a missed call.
  • Display features – Display calling number and calling name.
  • Calling number blocking – enable hiding of details (identification) by called party.
  • Call screening – enable subscriber to accept, reject, or forward call on the basis of list of received calls.

Voice Mail – Voice mail based on PSTN offers subscribers an alternative to answering machines. This voice mail facility is available even when called is busy.  The main voice mail services available to small business or residential users are noted below:

  • Voice messaging – subscribers can store and play recorded greetings and receive, review, and distribute them to outside users
  • Fax messaging – subscribers can send faxes and view them later

Business Services – They make major part of service provider’s revenue.  The services offered in this area are described below:

  • Virtual private voice networks
  • Centrex services
  • Call center services

Virtual Private Voice Networks – Virtual private voice networks in PSTNs are cost effective means of interconnecting voice traffic.  The other option is use of tie lines described in lesson “Describe PBX, trunk, tie lines”.  Virtual private voice network user’s access public networks by interconnecting private network facilities such as T1 lines, SS7 facilities, messaging and so on.  The identification of private virtual network using customer group IDs numbers is done using SS7.

Centrex Services – Centrex services enable service providers to small businesses voice and data services facilities similar to larger private solutions.  These services can be delivered using public switching infrastructure instead of costlier customer owned equipment.  This software is loaded in a switch which creates a virtual private business network.  This service can be used in the following manner are described below:

  • POTS – Designate and use these lines as Centrex lines.
  • Feature lines – Additional features/ functionalities provided over standard POTS lines, ISDN circuits, or switched 55/56 circuits.

Centrex offers a variety of features to its subscribers as noted below:

  • Call handling – such as call waiting, call forwarding, Call Park, hunt grouping, and voice mail.
  • Convenience features – such as automatic dial, speed dialing, ring again, calling line identification.
  • Customer dialing plans – customized plans for customer groups and abbreviated dialing for internal calls.
  • Management – Track and control aspects of a business services.
  • Security – such as line restrictions, employee authorization code, virtual access to private network and call record details to track abnormal activity.

Call Center Services – Call centers receive large volume of calls which need to be handled and distributed among multiple call agents intelligently.  The system should be intelligent to route calls to answering agents as required. Most commonly used implementations in a call center are described below:

Automatic Call Distribution (ACD) – these routes the calls to multiple answering stations, this enables tracking of usage patterns, traffic and agent performances. ACD systems support features such as queuing, answer in the order to call arrival and so on. They are located in central office.

Switch Computer Applications Interface (SCAI) – This enables switching of service providers switches to inform to the call center computer for call routing and handling.  SCAI store and coordinate incoming/outgoing calls and do routing of information.  They also interconnect caller party to an IVR (Interactive voice response) system to gather more information before routing call to answering agent.

Service Providers Services – These are internal functions provided in background to support PSTN users.  Such as number translation, routing, calling services and so on.  The following service provider services are noted below:

  • Database services
  • Operator services


Database Services – enable service providers to maintain, access, translate information to support special service and access numbers. Given below are examples of databases used in US frequently to provide following services as described below:

  • 800 number services – bulk usage rate for high level of incoming calls
  • 900 number services – providing information, public opinion polls etc
  • Calling card services – provide long distance services from any type of PSTN access
  • Authorization services – access to VPN services and security against fraud

Operator Services – The operator services offered are noted below:

Toll and assistance – Provide general assistance and handle long distance calls in PSTN networks. Operator based services like room rent information, call transfers etc.

Directory assistance – These services are available by dialing 3 digit national code or an area specific code. Searching for telephone numbers based on directory listings.

Billing services – such as collect calls, 3rd party billing, calling cards, and credit card services.

This concludes the CCNA Voice topic of services offered by a PSTN networks. PSTN offers many valuable services to its subscribers and service providers.