US 5566236 System and method for re-establishing a disconnected telephone communication

ABSTRACT – A system and method provides a tandem switch and call control/management platform to establish a telephone connection (bridge) between two parties, the telephone connection being initiated by one of the parties. The tandem switch connects a telecommunications system (Centrex or PBX) with cellular or wireless telephones. If either party is disconnected from the tandem switch, the telephone connection between the tandem switch and the remaining party is maintained. The telephone connection is re-established by instructing the tandem switch to reconnect the disconnected party to the maintained party.


The present invention relates in general to telecommunications equipment, and more particularly relates to methods and apparatus for interfacing wireless telecommunications terminals into an existing telecommunications network.


The telecommunications industry is currently undergoing a massive shift from wired telecommunications devices to wireless devices. The impetus for this transition is mobility, and reduced cost for system reconfiguration. The invention described herein fits the form and function of a “Personal Communications Services” as described in industry terms.

Cellular and other wireless telephones offer mobility, but at the expense of reliability, and interoperability. Additionally, wireless handsets provide unlimited access to the carrier of the telephone and lacks a mechanism for incorporating standard call coverage tools which are provided by the land based telecommunications systems (PBX or Centrex).

Unreliable Connections

Cellular and wireless handsets often lose connection. This is due to many technical reasons, including improper handoff between cells, signals below minimum thresholds, loss of battery power, etc. Typically, upon a lost connection, the caller and wireless user must re-establish communication manually.

Limited Access Screen

Most business telephone extensions are part of the organization’s coverage plan. This coverage plan may include, but is not limited to, live operator support, secretary or co-worker coverage, voicemail, etc. Most business phones have buttons for “Do Not Disturb”, and many have display screens to identify the (internal) calling party’s number and name, if available. Many executives have all arriving calls screened by their secretary, limiting direct access to the executive only for important calls.

The rapid growth in wireless device availability complicates the picture. Cellular telephones are not integrated with any organization’s telephone system, as the service is only sold and operated from off-premise locations reached via Central Office connections. A few other wireless devices have recently been made available to the Customer Premise Equipment marketplace to provide on-premise, wireless handset capability. These newer devices provide only simple analog interfaces, with a limited set of capabilities. Some manufacturers offer different levels of integration, but none provide direct determination of unknown callers, and the ability to “act on” the calls.

The lack of integration of wired and wireless telephones means that, for a cellular or cordless user:

No information about who is calling is transmitted to the handset.

There is no ability to screen the caller without answering.

Some wireless devices may be incapable of the “transfer” feature to move the call to another terminal.

Some wireless devices may not be capable of forwarding from the handset to another dialed number.

Calls can’t be forwarded or transferred from the on-premise networks to an off premise wireless devices phone.

The next generation devices, now just reaching the market, include “wireless Centrex” offerings, which use SS7 Common Channel Signaling protocols to integrate Centrex telephones into a Centrex partition. These services are not commonly available due to the slow deployment of the SS7 overlay signalling network into central offices.

Handoff/Transfer Between Cellular and Wired Telephones is Impossible

When a call arrives on a wireless device, the user of the device has the mobility to move from place to place. However, because wireless devices are not “secure”, and the quality of the voice may be less than that of a wired telephone, it may be desirable to switch the call to the nearest wired telephone, but only without losing connection. This has the added effect of saving money, since many wireless service providers charge for all air time.

The current art supports automatic handoff between different wireless systems which are homogeneous and support the cellular telephone protocol IS-41 (“roaming”). Unfortunately, handoffs to wired, or dissimilar wireless networks currently was impossible until the development of the current apparatus.


This invention uses a tandem switch to provide a “bridge” between two callers. In a preferred embodiment, the switch connects an organization’s telecommunications system (Centrex or PBX), and the cellular or wireless telephones. The switch facilitates a call coverage plan for an organization, and provides the ability to seize and dial a plurality of wireless networks as needed. An arriving call never totally leaves the fabric of the organization’s telecommunications system, providing access to all telecommunications coverage options, including transferring to additional parties, voicemail, etc.

Calls arriving or leaving an organization’s telecommunications system which connect to any wireless device can be routed to the PCS system described herein. When routed to and or through the PCS system, control of the call is obtained. This control includes the ability to treat each leg of the call as independent. The control of the call is used to implement the features described below.

The methods of interconnecting to the on-premise telecommunications equipment vary depending on the age and technical features of the equipment utilized. In a basic, preferred embodiment, the PCS actually holds both legs of the telephone calls across its call switching mechanism. The PCS will then have separate control over both legs, including the ability to detect near-end or far-end call release, rerouting of calls, and preemptive call interruption, call conferencing, and call joining.

In a more advanced embodiment, the call control is obtained and trunk and route optimization is performed using the Advanced Intelligent Network architecture described in the CCITT Recommendations and Bellcore documents listed below:

CCITT Q.700: Introduction to CCITT SS7

CCITT Q.775: Guidelines for Using Transaction Capabilities

CCITT Q.724: Signaling Procedures

CCITT Q.761: Functional Description of the ISDN User Part of SS7

TR-NWT-000246 Issue 2, Revision 2: Bell Communications Research Specification of Signaling System Number 7

SR-NPL-001509 Issue 1: Advanced Intelligent Network Release 1 Proposal

SR-NPL-001623 Issue 1: Advance Intelligent Network Release 1 Network and Operations Plan

The control of the call is retained through the use of “triggers” which are set for both arriving calls, and calls in progress. The “triggers” fire when certain events occur, and control of the call is returned to the PCS system.


This invention provides a connection safe guard by waiting for a disconnect to occur on either the caller side, and/or the wireless users side. If the wireless user disconnects first, then this trigger event will return control of the remaining caller to a voice processing system. The voice processing system will optionally prompt the caller to indicate whether reconnection is desired. If so, the voice processing system will re-initiate the call to the wireless device, and/or wait for the caller to call back.

If the call originated on a wireless device, and the wireless device disconnects, then the remaining caller is prompted to see if reconnection is desired. If so, the caller is placed on hold, waiting for the wireless device to call back. Optionally, an alert is transmitted which could be a page message or cellular data message sent to the wireless user to notify of the disconnected caller waiting for reconnection.

If the wireless device address (location and/or dialed number) is not known then the caller can be placed on hold, awaiting the wireless handset’s reconnection attempt. This permits the invention to operate correctly on all wireless devices, including those which have no common signaling with the PCS.

This above invention will be called “CellGuard” throughout the following document. This invention is not limited to cellular or wireless telephones, and will provide the same functionality on all types of telecommunications equipment. Additionally, the features can be provided using different technologies. Illustrative methods of embodying this invention are described herein.


The objective of the invention is to provide call screening through multiple means so that the wireless user can determine who is calling, the nature of the call, etc. before speaking to the caller. The unwanted screened call can be rerouted, placed into a call coverage option (including voicemail), terminated, or handled any way desired by the on-premise telecommunications system and selected by the user. Throughout this document, the term “CellScreen” will be used to describe this embodiment.


The hand-off from wireless to wired and vice-versa can be performed using the invention described in this document. This hand-off can occur once, or several times during the course of a single connection. The hand-off is completed by having a new call “steal” the call away from the conversation in progress. This provides the mechanism for unlimited hand-offs between dissimilar networks.


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