A callback mechanism for Private Telecommunications Networks

Private Telecommunications Networks (PTNs), such as enterprise telephony and Voice over IP (VoIP), have been widely deployed. A common limitation of PTNs is that PTN users are usually not assigned with Public Switched Telephone Network (PSTN) E.164 numbers. A PTN user is usually identified by an extension number or a URI (universal resource identifier). When a PTN user initiates a call to a PSTN user, it is not possible to use the PTN calling party's identifier as the caller ID; instead, an E.164 number of the PBX or VoIP gateway is used. As a result, the PSTN called party cannot use the caller ID received to call back to the PTN calling party later, if the PTN user's extension number or URI is unknown. This paper proposes a callback table approach to solve this problem. We describe how the call-out information is stored into and retrieved from a callback table maintained in the PBXs or VoIP gateways to provide callback service. Numeric analysis has been performed to evaluate the performance of this callback solution. The numeric results indicate that the callback method provides a satisfactory solution when there is sufficient voice traffic served by a VoIP gateway.