# Patent

## Title
System and method for transitioning a communication session between networks that are not commonly controlled

## Abstract

Provided are a system and method for network transitions. In one example, the method includes receiving messages from a mobile device indicating that the mobile device is in a session initiation protocol (SIP) wireless hotspot and that the mobile device is engaged in a communication session via a cellular network while in the SIP wireless hotspot. The cellular network is instructed to add the mobile device to the communication session via the SIP wireless hotspot. An affirmative response is received from the cellular network and a third message is sent to the mobile device to notify the mobile device of the affirmative response.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent Application 60/976,014, filed on Sep. 28, 2007.


INCORPORATED BY REFERENCE
The present application incorporates the following applications by reference: U.S. patent application Ser. No. 11/214,648, filed on Aug. 30, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 11/081,068, filed on Mar. 15, 2005, which claims the benefit of U.S. Provisional Patent Ser. Nos. 60/583,536, filed Jun. 29, 2004, 60/628,183, filed Nov. 15, 2004, and 60/628,291, filed Nov. 17, 2004.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying Drawings in which:
FIG. 1 is a diagram of one embodiment of a communications environment.
FIG. 2 is a sequence diagram illustrating one embodiment of a message flow that may be used to transition a communication session from a cellular network to a hotspot within the environment of FIG. 1.
FIG. 3 is a sequence diagram illustrating one embodiment of a message flow that may be used to transition a communication session from a hotspot to a cellular network within the environment of FIG. 1.
FIG. 4 is a diagram of another embodiment of a communications environment.
FIG. 5 is a flow chart illustrating one embodiment of a method that may be used to transition a communication session from a cellular network to a hotspot within the environment of FIG. 1.
FIG. 6 is a flow chart illustrating one embodiment of a method that may be used to transition a communication session from a hotspot to a cellular network within the environment of FIG. 1.



DETAILED DESCRIPTION
The present disclosure is directed to systems and methods for networked communications. It is understood that the following disclosure provides many different embodiments or examples. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.
Referring to FIG. 1, in one embodiment, a communications environment 100 includes a mobile device 102 capable of wireless communications. The mobile device 102 may be any device capable of receiving and sending information via a wireless network, and may include processing capabilities to handle such information. Such devices include mobile computers, cell phones, personal digital assistants (PDAs), and pagers. In some embodiments, the mobile device 102 may be an endpoint as described in previously incorporated U.S. patent application Ser. No. 11/081,068. In the present example, the mobile device 102 has the capability to communicate via both Wi-Fi and cellular networks. For example, the mobile device 102 may include both a Wi-Fi interface and a cellular interface.
The environment 100 includes a mobile virtual network operator (MVNO) 104. The MVNO 104 may lease or otherwise license wireline and/or wireless bandwidth from companies who own such resources. Accordingly, the MVNO 104 may provide communication services to customers (e.g., the mobile device 102) using network resources not controlled by the MVNO itself, although some network resources may be controlled by the MVNO. Because of the license costs associated with leasing bandwidth from other providers, it may be expensive for the MVNO to provide access to networks that it does not control.
For purposes of example, the environment 100 also includes a data packet network 106 (e.g., the Internet), a public switched telephone network (PSTN) 108, a hotspot 110, and a cellular network 112. The hotspot 110 may use many different protocols to link the mobile device 102 to the data packet network 106. Exemplary network types for the hotspot 110 include wideband digital communication systems such as wireless local area networks (LANs) (e.g., IEEE 802.11a and 802.11g Wi-Fi networks) and WiMax wireless metropolitan area networks (MANs) (e.g., IEEE 802.16 networks). The PSTN 108 may provide SS7 and/or TDM functionality, and may also provide trunk line access for MVNO 104 in some embodiments. Generally, the PSTN 108 may use the cellular network 112 to provide mobility to users. The PSTN 108 may include one or more network gateways, which are referred to herein interchangeably with the PSTN. The cellular network 112 may represent many different cellular communication systems, including Global System for Mobile communications (GSM) and/or code division multiple access (CDMA) systems. In some embodiments, one or more of the networks 106, 108, 110, and 112 may include hybrid peer-to-peer network functionality as described in previously incorporated U.S. patent application Ser. No. 11/081,068.
The hotspot 110 may include an access point 114 that provides wireless access to an area defined, for purposes of example, by boundary 116 and is coupled to data packet network 106. The cellular network 112 may include a base station 118 that provides wireless access to an area defined, for purposes of example, by boundary 120 and is coupled to PSTN 108. It is understood that both the hotspot 110 and cellular network 112 may have additional components (e.g., home location registers (HLRs), visitor location registers (VLRs), mobile switching centers (MSCs), and base station controllers (BSCs)) that are not shown in FIG. 1. An overlapping zone 122 represents an area where the hotspot 110 and cellular network 112 overlap and where the mobile device 102 has access to both. It is understood that the shape and size of the areas covered by the hotspot 110, cellular network 112, and zone 122 are for purposes of illustration only and may differ greatly from those shown.
The environment 100 also includes a network transition gateway 124 that is under the control of the MVNO 104. It is understood that the MVNO 104 may or may not own or otherwise operate the network transition gateway 124, but may have a certain amount of control over the gateway with respect to customers of the MVNO. In the present example, the network transition gateway 124 is positioned between the MVNO 104 and the PSTN 108 and forms the outer boundary of the MVNO's control. In other embodiments, the MVNO's control may extend to other networks, such as the PSTN 108 (or other networks taking the place of the PSTN in FIG. 1). Accordingly, in the present embodiment, the MVNO 104 is not able to perform call setup or routing actions in the cellular network, but can only send a request to the cellular network to establish, breakdown, or route a call. Due to the cost issues of leasing bandwidth from the cellular network 112, the MVNO 104 may desire to minimize its customers use of the cellular network and so may desire to route calls through other channels (e.g., the hotspot 110) whenever possible.
The network transition gateway 124 may include HLR and/or VLR functionality from a Session Initiation Protocol (SIP) perspective. For example, the network transition gateway 124 may have a list of all mobile devices belonging to customers of the MVNO 104 in the HLR and corresponding SIP information. For example, the HLR may include a SIP address for the mobile device 102 (e.g., john@sip.mvno.com) that is cross referenced to the phone number of the mobile device (e.g., 972-555-1212). This cross referencing enables the network transition gateway 124 to move between SIP messaging and phone communications with respect to the mobile device 102 as will be described below in greater detail. The network transition gateway 124 may also include VLR functionality that registers the mobile device's presence when the mobile device 102 enters the hotspot 110 and is available for SIP messaging. When the mobile device 102 leaves the hotspot 110 or is turned off, its entry in the VLR is removed to indicate that the mobile device is not available for SIP messaging. The network transition gateway 124 may also include other routing and control elements that are not SIP based. It is understood that the SIP VLR and SIP HLR functionality, as well as the other routing and control elements of the network transition gateway 124, may be located elsewhere in other embodiments. Non-MVNO networks, such as the cellular network 112, may be instructed to route all call requests initiated by subscribers of the MVNO to an aggregator within the MVNO, such as the network transition gateway 124.
Accordingly, the MVNO 104 may use functionality provided by the network transition gateway 124 to determine how to service a communication session (e.g., a phone call or data transfer) and to perform transitions between networks when needed. In the present example, the network transition gateway 124 remains in control of the communication session for the duration of the session regardless of the network used by the mobile device 104. It is noted that this includes calls initiated by the mobile device 104 in the cellular network 112 even though the MVNO 104 has no control over the cellular network, as the network transition gateway 124 functions as a bridge to the cellular network 112 for calls that use the cellular network.
The mobile device 102 may be involved in a number of scenarios that may be viewed based on the accessibility of the hotspot 110 and cellular network 112. As a general rule, when the hotspot is available, the mobile device 102 is routed through the hotspot rather than the cellular network 112.
Referring to FIG. 2, in one embodiment, a sequence diagram 200 illustrates a sequence of messages that may occur when the mobile device 102 is originally serviced by the cellular network 112 and moves into a location where it can be serviced by the hotspot 110. In the present example, the mobile device 102 has a current call that was established and serviced by the cellular network 112. As the mobile device 102 is now located in the hotspot 110, it is desirable to switch the call to the hotspot 110. Accordingly, the mobile device 102 may contain software that identifies the accessibility of the hotspot 110, identifies the ongoing call, and communicates these facts to the network transition gateway 124.
Although not shown, to initiate the call in the cellular network 112, the mobile device 102 sends a call setup request to the cellular network as would ordinarily occur. For example, an MSC in the cellular network 112 may receive the call request and look up the subscriber ID associated with the mobile device. Due to instructions by the MVNO, the MSC will identify that all calls initiated by mobile devices with subscriber IDs identifying them as MVNO subscribers are to be routed to an aggregator (e.g., the network transition gateway 124) without any other action being taken (e.g., the MSC is to route the request and should not set up the call itself). Accordingly, the MSC will route the call request to the network transition gateway 124, which may then determine how to set up the call. It is noted that if the call is terminated to a subscriber within the cellular network 112, the network transition gateway 124 may communicate with the cellular network to establish the second leg of the call between the network transition gateway and the cellular network, but does not relinquish control of the first call leg. This enables the network transition gateway 124 to maintain control of the call even when the call originates and terminates within the cellular network 112.
In step 202, once the mobile device 102 enters the hotspot 110, the mobile device registers with the network transition gateway 124, which may enter the mobile device 102 into the SIP VLR. The registration message from the mobile device 102 notifies the network transition gateway 124 of the presence of the mobile device in a SIP messaging environment.
The present disclosure, both in the current embodiment and following embodiments, refers to various messages. It is understood that these are simply examples and are not intended to limit the disclosure to the particular messages described. In step 204, the mobile device 102 sends an INVITE/SDP message to notify the network transition gateway 124 that there is an ongoing call and to provide information to the network transition gateway regarding the call.
In step 206, the network transition gateway 124 sends an INVITE message to the PSTN 108 (e.g., to a network gateway 109 within the PSTN 108) requesting that the PSTN add the mobile device 102 to the call (via the hotspot 110). The INVITE may include additional information, such as a hold request to inform the PSTN 108 that it is to hold the call. In step 208, the PSTN network gateway 109 responds to the network transition gateway 124, which in turn responds to the mobile device 102 in step 210. In step 212, the PSTN network gateway 109 instructs the cellular network 112 to drop the call and, in step 214, the cellular network drops its leg of the call with the mobile device 102 and ceases its participation.
Accordingly, although the call was initially serviced using a call leg between the mobile device 102 and the cellular network 112, the call is moved from the cellular network to a Wi-Fi connection via the hotspot 110. Before, during, and after the transition, the call is controlled by the MVNO via the network transition gateway 124.
Referring to FIG. 3, in another embodiment, a sequence diagram 300 illustrates a sequence of messages that may occur when the mobile device 102 is originally serviced by the hotspot 110 and moves into a location where it can be serviced by the cellular network 112 but not the hotspot 110. In the present example, the mobile device 102 has a current call that was established and serviced via the hotspot 110. As the mobile device 102 is now leaving the coverage area of the hotspot 110, it is desirable to switch the call to the cellular network 110.
Accordingly, the mobile device 102 may contain software that identifies that it is leaving the hotspot 110 (e.g., due to location information or loss of signal strength) and may send an INVITE message with hold information to the network transition gateway 124 in step 302. The network transition gateway 124 may be configured to interpret the hold as indicating that the mobile device 102 is moving out of the hotspot 110 and/or into the cellular network 112.
In step 304, the network transition gateway 124 sends a request to the PSTN gateway to perform a call setup from the mobile device 102 via the network transition gateway 124 to the cellular network. In step 306, the mobile device 102 may send a message to the network transition gateway 124 to deregister from the network transition gateway. The network transition gateway 124 may then remove the mobile device 102 from the SIP VLR and tear down the IP channel supporting the initial call leg to the mobile device 102 in step 308.
In step 310, the PSTN network gateway 109 sends a setup request to the cellular network 112 to set up a call for the mobile device 102 and, in step 312, the cellular network establishes the connection with the mobile device. In step 314, the mobile device 102 may send information, such as dual tone multi frequency (DTMF) trigger information, to the cellular network 112 to inform the cellular network that the mobile device has switched over to the cellular network. For example, the mobile device 102 may request that its user press a key that is then sent to the cellular network 112 or the mobile device may automatically send such information to the cellular network.
Accordingly, although the call was initially serviced by a call leg between the hotspot 110 and the mobile device 102, the call is moved from the hotspot to a cellular connection via the cellular network 112. Before, during, and after the transition, the call is controlled by the MVNO via the network transition gateway 124.
It is understood that the actual transition process may vary depending on the capabilities of the mobile device 102. For example, the mobile device 102 may have the capability to handle simultaneous connections via its Wi-Fi and cellular interfaces. In this case, the mobile device 102 may put one call leg on hold, set up another call leg, and then join the legs before dropping the initial leg. In such scenarios, the initial call leg may not be broken down before the next leg is established. Accordingly, the actual process used to transition the mobile device 102 from the hotspot 110 to the cellular network 112 or vice versa may depend on the mobile device itself. However, regardless of the capabilities of the mobile device 102, the MVNO maintains control of the call before, during, and after the transition, via the network transition gateway 124.
Although only transitions between the hotspot 110 and the cellular network 112 are illustrated, it is understood that the network transition gateway 124 may handle network transitions between other types of networks supported by the mobile device 102. For example, the network transition gateway 124 may be configured to locate a route for a call based on cost and/or other factors and may handle whatever network transitions are needed to setup the call using the located route.
It is understood that both incoming and outgoing call involving the mobile device 102 may be controlled by the network transition gateway 124 regardless of whether the mobile device is using the hotspot 110 or the cellular network 112. Once the call is established, the mobile device 102 may move between the cellular network 112 and the hotspot 110 with the network transition gateway 124 handling the establishment and tear down of call legs as needed.
Referring to FIG. 4, in yet another embodiment, two endpoints 402 and 404 are located in networks 406 and 408, respectively. The networks 406 and 408 (which may be a single network in some embodiments) are controlled by MVNO 104. The endpoints may be similar or identical to those described in previously incorporated U.S. patent application Ser. No. 11/081,068. One or both of the endpoints 402 and 406 may be mobile devices as previously described. Although not shown, other network components (e.g., an access server and a STUN server as described in U.S. patent application Ser. No. 11/081,068) may be included in the MVNO 104 to handle the endpoints 402 and 404. For example, access server functionality may be included in the network transition gateway 124 or provided by a separate server.
In the present example, signaling between the endpoints may be handled by the network transition gateway 124 as previously described. However, due to the peer-to-peer functionality of the endpoints 402 and 404, the two endpoints may establish a direct media path through the data packet network 106 that does not go through the MVNO 104 or network transition gateway 124. This peer-to-peer process is described in detail in U.S. patent application Ser. No. 11/081,068.
Referring to FIG. 5, in yet another embodiment, a method 500 may be used to transition a mobile device (e.g., the mobile device 102 of FIG. 1) from a cellular network (e.g., the cellular network 112 of FIG. 1) to a hotspot (e.g., the hotspot 110 of FIG. 1). In step 502, a communication session may be established for the mobile device 102 via a network transition gateway (e.g., the network transition gateway 124 of FIG. 1), where the call is initially established at least partially within the cellular network 112 where the network transition gateway has no control. In step 504, the method 500 identifies that the mobile device 102 can access the hotspot 110 and, in step 506, identifies that the mobile device is currently engaged in the communication session that is using cellular network resources, where the mobile device is communicating via the cellular network 112 using a first call leg established between the mobile device and the cellular network. In step 508, the network transition gateway 124 establishes a second call leg via the hotspot 110 between the mobile device 102 and a network gateway (e.g., a network gateway in the PSTN 108) located outside of the cellular network 112 and, in step 510, terminates the first call leg. It is understood that the network transition gateway 124 may simply initiate certain actions, such as terminating the first call leg, and may not perform the termination itself.
Referring to FIG. 6, in yet another embodiment, a method 600 may be used to transition a mobile device (e.g., the mobile device 102 of FIG. 1) from a hotspot (e.g., the hotspot 110 of FIG. 1) to a cellular network (e.g., the cellular network 112 of FIG. 1). In step 602, a communication session is established for the mobile device 102 within the hotspot 110 via a network transition gateway (e.g., the network transition gateway 124 of FIG. 1). In step 604, the method 600 identifies that the mobile device 102 is losing access to the hotspot 110 and, in step 606, identifies that the mobile device is currently engaged in the communication session and needs to access cellular network resources over which the network transition gateway 124 has no control, where the mobile device is communicating via the hotspot using a first call leg established between the network transition gateway and the mobile device. In step 608, the network transition gateway 124 establishes a second call leg between the mobile device 102 and the cellular network 112, where the second call leg passes through the network transition device. In step 610, the first call leg is terminated. It is understood that the network transition gateway may simply initiate certain actions, such as terminating the first call leg, and may not perform the termination itself.
While the preceding description shows and describes one or more embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present disclosure. For example, various steps illustrated within a particular sequence diagram may be combined or further divided, and some steps may be performed in a different order than that shown. In addition, steps described in one diagram may be incorporated into another diagram. Furthermore, the described functionality may be provided by hardware and/or software, and may be distributed or combined into a single platform. Additionally, functionality described in a particular example may be achieved in a manner different than that illustrated, but is still encompassed within the present disclosure. Therefore, the claims should be interpreted in a broad manner, consistent with the present disclosure.

## Claims

1. A method comprising:
receiving, by a network transition gateway controlled by a mobile virtual network operator (MVNO), a first message directly from a mobile device indicating that the mobile device is in a session initiation protocol (SIP) wireless hotspot, wherein the first message is received directly from the mobile device by the network transition gateway and does not pass through a cellular network, and wherein the mobile device is present in a session initiation protocol (SIP) home location register (HLR) of the network transition gateway that lists mobile devices belonging to customers of the MVNO;
receiving, by the network transition gateway, a second message directly from the mobile device indicating that the mobile device is engaged in a communication session with a second device using a first call leg via the cellular network while in the SIP wireless hotspot, wherein the second message is received directly from the mobile device by the network transition gateway and does not pass through the cellular network, and wherein the MVNO and the network transition gateway have no control over the cellular network and wherein the network transition gateway is configured to send only requests for establishing, routing, and breaking down a call to the cellular network;
requesting, by the network transition gateway, that a public switched telephone network (PSTN) gateway add the mobile device to the communication session using a second call leg via the SIP wireless hotspot and terminate the first call leg, wherein the PSTN gateway is separate from the cellular network and is not controlled by the MVNO;
receiving, by the network transition gateway, an affirmative response from the PSTN gateway; and
sending, by the network transition gateway, a third message to the mobile device to notify the mobile device of the affirmative response.

2. The method of claim 1 further comprising:
receiving a fourth message from the cellular network via the PSTN gateway indicating that the mobile device has requested that the cellular network establish the communication session;
determining that the mobile device is attempting to establish the communication session with a user within the cellular network;
establishing a third call leg with the user via the cellular network;
establishing the first call leg with the mobile device via the cellular network; and
joining the third and second call legs to establish the communication session.

3. The method of claim 1 further comprising instructing the PSTN gateway to place the communication session on hold until the mobile device is added to the communication session via the SIP wireless hotspot.

4. The method of claim 1 further comprising:
receiving, by the network transition gateway, a fourth message from the mobile device indicating that it is leaving the SIP wireless hotspot and is engaged in the communication session; and
sending, by the network transition gateway, a request message to the cellular network via the PSTN gateway to set up a communication channel with the mobile device to continue the communication session via the cellular network, wherein the network transition gateway maintains control of the communication session after the mobile device transitions to the cellular network over which the network transition gateway has no control, and wherein the second call leg is terminated.

5. The method of claim 1 further comprising adding, by the network transition gateway, the mobile device to a visitor location register (VLR) of the network transition gateway to indicate that the mobile device is in the SIP wireless hotspot after receiving the second message.

6. The method of claim 1 wherein signaling between the mobile device and the second device for the communication session passes through the network transition gateway and wherein data for the communication session passes directly between the mobile device and the second device without passing through the network transition gateway.

7. A method for execution on a mobile device comprising:
maintaining, by the mobile device, a communication session with a second device via a first call leg over a cellular network;
detecting, by the mobile device, the presence of a Session Initiation Protocol (SIP) wireless hotspot while the communication session is being maintained;
notifying, by the mobile device, a network transition gateway of the presence of the SIP wireless hotspot and the communication session being maintained;
receiving, by the mobile device, instructions from the network transition gateway to establish a second call leg with a public switched telephone network (PSTN) gateway via the SIP wireless hotspot;
establishing, by the mobile device, the second call leg; and
continuing, by the mobile device, the communication session via the second call leg over the SIP wireless hotspot, wherein signaling between the mobile device and the second device passes through the network transition gateway and wherein data between the mobile device and the second device is peer-to-peer and does not pass through the network transition gateway.

8. The method of claim 7 further comprising joining the first and second call legs.

9. The method of claim 7 further comprising terminating the first call leg after establishing the second call leg.

10. The method of claim 7 further comprising:
sending a request to the cellular network to establish the communication session;
receiving call setup information from the network transition gateway; and
responding to the call setup information to establish the communication session with the cellular network via the network transition gateway.

11. The method of claim 7 further comprising:
sending a request to the cellular network to establish the communication session;
receiving call setup information from the cellular network; and
responding to the call setup information to establish the communication session with the cellular network.

12. The method of claim 11 further comprising sending dual tone multi frequency (DTMF) trigger information to the cellular network to inform the cellular network that the mobile device has switched to the cellular network.

13. The method of claim 7 further comprising:
performing an authentication process with an access server;
obtaining, following the authentication process, a profile containing a list of devices of that are currently available to communicate with the mobile device, wherein each of the devices on the list has previously granted the mobile device permission to communicate with that device and wherein the second device is on the list of devices; and
prior to maintaining the communication session, sending the second device a notification message after obtaining the profile, wherein the notification message informs the second device that the mobile device is available.

14. A method comprising:
receiving, by a network transition gateway via a public switched telephone network (PSTN) gateway, a first message from a cellular network indicating that a mobile first device has sent a call setup request to the cellular network for a communication session with a second device, wherein the network transition gateway is operated by a mobile virtual network operator (MVNO) that has no control over the cellular network and wherein the first mobile device is present in a session initiation protocol (SIP) home location register (HLR) of the network transition gateway;
communicating, by the network transition gateway via the PSTN gateway, with the cellular network to establish the call for the first device via the cellular network using a first call leg between the first device and the cellular network, wherein the network transition gateway maintains a second call leg with the cellular network for signaling to control the communication session, and wherein the network transition gateway is configured to send only requests for establishing, routing, and breaking down a call to the cellular network;
receiving, by the network transition gateway, a second message from the first device indicating that the first device has entered a SIP wireless hotspot, wherein the second message is received while the communication session is maintained through the cellular network;
entering, by the network transition gateway, the first device in a register of the network transition gateway to indicate that the first device is in the SIP wireless hotspot; and
sending, by the network transition gateway to the PSTN gateway, a third message requesting that the PSTN gateway add the first device to the call by establishing a third call leg with the first device via the SIP wireless hotspot and that the first call leg between the first device and the cellular network be terminated, wherein the network transition gateway maintains control of the call after the call is moved from the cellular network to the SIP wireless hotspot.

15. The method of claim 14 wherein signaling by the first and second devices for the communication session passes through the network transition gateway and wherein data for the communication session passes directly between the first and second devices without passing through the network transition gateway.

16. The method of claim 14 wherein both signaling and data for the communication session passes through the network transition gateway.

17. The method of claim 14 further comprising
receiving, by the network transition gateway, a third message from the first device indicating that the first device is leaving the SIP wireless hotspot;
communicating, by the network transition gateway via the PSTN gateway, with the cellular network to maintain the call for the first mobile device via the cellular network using a fourth call leg between the first device and the cellular network, wherein the network transition gateway maintains control of the communication session after the call is moved;
removing, by the network transition gateway, the first device from the register of the network transition gateway to indicate that the first device is not in a SIP wireless hotspot; and
instructing, by the network transition gateway, the first device to tear down the third call leg with the SIP wireless hotspot.

18. The method of claim 14 wherein the sending, by the network transition gateway to the PSTN gateway, the third message requesting that the PSTN gateway add the first device to the call includes requesting that the PSTN gateway place the call on hold.

19. The method of claim 14 wherein the HLR of the network transition gateway includes a SIP address and a telephone number of the first device.