Patent Publication Number: US-2023156865-A1

Title: Breakout gateway control function number modification based on sip invite header information

Description:
BACKGROUND 
     Historically, mobile phones have provided voice call services to users over a circuit-switched-style network, such as the public switched telephone network (PSTN), rather than over an IP packet-switched network. To enable delivery of IP multimedia services, the IP Multimedia Subsystem or IP Multimedia Core Network Subsystem (IMS) was introduced as an architectural framework. The IMS uses the Session Initiation Protocol (SIP), a signaling protocol, to initiate, maintain, and terminate real-time sessions that include voice, video and messaging applications. The SIP is used for signaling and controlling multimedia communication sessions in applications of Internet telephony for voice and video calls, in private IP telephone systems, in instant messaging over IP networks as well as voice calling over Long-Term Evolution (VoLTE) or New Radio (VoNR). 
     IMS terminals such as mobile phones and other user equipment (UEs) can register directly on IMS to make calls, even when they are roaming in another network or country. The calls are routed across a network of switching systems, sometimes owned by different network operators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed descriptions of implementations of the present invention will be described and explained using the accompanying drawings. 
         FIG.  1    is a block diagram that illustrates a wireless communications system. 
         FIG.  2    illustrates an example architecture of an Internet Protocol (IP) Multimedia Subsystem (IMS) in which aspects of the present invention can operate. 
         FIG.  3    is a flowchart that illustrates a process for generating a SIP INVITE URI based on a P-Access-Network-Info (PANI) header information. 
         FIG.  4    is a block diagram that illustrates an example toll free number modification. 
         FIG.  5    is a flow diagram that illustrates number modification for an IMS-originated toll-free call. 
         FIG.  6    is a flow diagram that illustrates number modification for an IMS-originated directory assistance call. 
         FIG.  7    is a block diagram that illustrates an example of a computer system in which at least some operations described herein can be implemented. 
     
    
    
     The technologies described herein will become more apparent to those skilled in the art from studying the Detailed Description in conjunction with the drawings. Embodiments or implementations describing aspects of the invention are illustrated by way of example, and the same references can indicate similar elements. While the drawings depict various implementations for the purpose of illustration, those skilled in the art will recognize that alternative implementations can be employed without departing from the principles of the present technologies. Accordingly, while specific implementations are shown in the drawings, the technology is amenable to various modifications. 
     DETAILED DESCRIPTION 
     In one aspect of the disclosed technology, a Breakout Gateway Control Function (BGCF) determines a geographic location information (e.g., a major trading area (MTA)) from a SIP INVITE header information (e.g., from a P-Access-Network-Info (PANI) header information). The BGCF modifies the SIP URI in the SIP INVITE request by adding, deleting, or modifying values (e.g., prepending a number prefix) to generate a SIP URI that is based on the geographic location information. For example, for two SIP INVITES from two users dialing the same telephone number (e.g., the same toll-free number) from different geographic locations, the BGCF modifies the SIP URIs to provide two different SIP URIs with different prefixes that are based on the user locations. The two SIP URIs are routed to different destinations based on the caller&#39;s location. This allows different callers to use the same dialing number (e.g., the same 1-800 toll-free number or the same 411 directory-assistance number) to reach different destinations where each destination corresponds to the caller&#39;s location (e.g., a caller reaches the destination better suited to handle the caller&#39;s inquiries). 
     In another aspect of the disclosed technology, the BGCF sends the modified SIP URIs to an Interconnect Session Border Controller (ISBC) without traversing any intermediate network nodes or functions and/or without any intervening processing of the modified SIP URI that would further modify the SIP INVITE URIs. 
     The description and associated drawings are illustrative examples and are not to be construed as limiting. This disclosure provides certain details for a thorough understanding and enabling description of these examples. One skilled in the relevant technology will understand, however, that the invention can be practiced without many of these details. Likewise, one skilled in the relevant technology will understand that the invention can include well-known structures or features that are not shown or described in detail, to avoid unnecessarily obscuring the descriptions of examples. 
     Wireless Communications System 
       FIG.  1    is a block diagram that illustrates a wireless telecommunication system  100  (“system  100 ”) in which aspects of the disclosed technology are incorporated. The system  100  includes base stations  102 - 1  through  102 - 4  (also referred to individually as “base station  102 ” or collectively as “base stations  102 ”). A base station is a type of network access node (NAN) that can also be referred to as a cell site, a base transceiver station, or a radio base station. The system  100  can include any combination of NANs including an access point, radio transceiver, gNodeB (gNB), NodeB, eNodeB (eNB), Home NodeB or eNodeB, or the like. In addition to being a WWAN base station, a NAN can be a WLAN access point, such as an Institute of Electrical and Electronics Engineers (IEEE) 802.11 access point. 
     The NANs of a network formed by the system  100  also include wireless devices  104 - 1  through  104 - 8  (referred to individually as “wireless device  104 ” or collectively as “wireless devices  104 ”) and a core network  106 . The wireless devices  104 - 1  through  104 - 8  can correspond to or include network entities capable of communication using various connectivity standards. For example, a 5G communication channel can use millimeter wave (mmW) access frequencies of 28 GHz or more. In some implementations, the wireless device  104  can operatively couple to a base station  102  over an LTE/LTE-A communication channel, which is referred to as a 4G communication channel. 
     The core network  106  provides, manages, and controls security services, user authentication, access authorization, tracking, Internet Protocol (IP) connectivity, and other access, routing, or mobility functions. The base stations  102  interface with the core network  106  through a first set of backhaul links  108  (e.g., S1 interfaces) and can perform radio configuration and scheduling for communication with the wireless devices  104  or can operate under the control of a base station controller (not shown). In some examples, the base stations  102  can communicate, either directly or indirectly (e.g., through the core network  106 ), with each other over a second set of backhaul links  110 - 1  through  110 - 3  (e.g., X 1  interfaces), which can be wired or wireless communication links. 
     The base stations  102  can wirelessly communicate with the wireless devices  104  via one or more base station antennas. The cell sites can provide communication coverage for geographic coverage areas  112 - 1  through  112 - 4  (also referred to individually as “coverage area  112 ” or collectively as “coverage areas  112 ”). The geographic coverage area  112  for a base station  102  can be divided into sectors making up only a portion of the coverage area (not shown). The system  100  can include base stations of different types (e.g., macro and/or small cell base stations). In some implementations, there can be overlapping geographic coverage areas  112  for different service environments (e.g., Internet-of-Things (IoT), mobile broadband (MBB), vehicle-to-everything (V2X), machine-to-machine (M2M), machine-to-everything (M2X), ultra-reliable low-latency communication (URLLC), machine-type communication (MTC)), etc. 
     The system  100  can include a 5G network and/or an LTE/LTE-A or other network. In an LTE/LTE-A network, the term eNB is used to describe the base stations  102  and in 5G new radio (NR) networks, the term gNBs is used to describe the base stations  102  that can include mmW communications. The system  100  can thus form a heterogeneous network in which different types of base stations provide coverage for various geographical regions. For example, each base station  102  can provide communication coverage for a macro cell, a small cell, and/or other types of cells. As used herein, the term “cell” can relate to a base station, a carrier or component carrier associated with the base station, ora coverage area (e.g., sector) of a carrier or base station, depending on context. 
     A macro cell generally covers a relatively large geographic area (e.g., several kilometers in radius) and can allow access by wireless devices with service subscriptions with a wireless network service provider. As indicated earlier, a small cell is a lower-powered base station, as compared with a macro cell, and can operate in the same or different (e.g., licensed, unlicensed) frequency bands as macro cells. Examples of small cells include pico cells, femto cells, and micro cells. In general, a pico cell can cover a relatively smaller geographic area and can allow unrestricted access by wireless devices with service subscriptions with the network provider. A femto cell covers a relatively smaller geographic area (e.g., a home) and can provide restricted access by wireless devices having an association with the femto cell (e.g., wireless devices in a closed subscriber group (CSG), wireless devices for users in the home). A base station can support one or multiple (e.g., two, three, four, and the like) cells (e.g., component carriers). All fixed transceivers noted herein that can provide access to the network are NANs, including small cells. 
     The communication networks that accommodate various disclosed examples can be packet-based networks that operate according to a layered protocol stack. In the user plane, communications at the bearer or Packet Data Convergence Protocol (PDCP) layer can be IP-based. A Radio Link Control (RLC) layer then performs packet segmentation and reassembly to communicate over logical channels. A Medium Access Control (MAC) layer can perform priority handling and multiplexing of logical channels into transport channels. The MAC layer can also use Hybrid ARQ (HARQ) to provide retransmission at the MAC layer, to improve link efficiency. In the control plane, the Radio Resource Control (RRC) protocol layer provides establishment, configuration, and maintenance of an RRC connection between a wireless device  104  and the base stations  102  or core network  106  supporting radio bearers for the user plane data. At the Physical (PHY) layer, the transport channels are mapped to physical channels. 
     As illustrated, the wireless devices  104  are distributed throughout the system  100 , where each wireless device  104  can be stationary or mobile. A wireless device can be referred to as a mobile station, a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a handheld mobile device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a mobile client, a client, or the like. Examples of a wireless device include user equipment (UE) such as a mobile phone, a personal digital assistant (PDA), a wireless modem, a handheld mobile device (e.g., wireless devices  104 - 1  and  104 - 2 ), a tablet computer, a laptop computer (e.g., wireless device  104 - 3 ), a wearable (e.g., wireless device  104 - 4 ). A wireless device can be included in another device such as, for example, a drone (e.g., wireless device  104 - 5 ), a vehicle (e.g., wireless device  104 - 6 ), an augmented reality/virtual reality (AR/VR) device such as a head-mounted display device (e.g., wireless device  104 - 7 ), an IoT device such as an appliance in a home (e.g., wireless device  104 - 8 ), a portable gaming console, or a wirelessly connected sensor that provides data to a remote server over a network. 
     A wireless device can communicate with various types of base stations and network equipment at the edge of a network including macro eNBs/gNBs, small cell eNBs/gNBs, relay base stations, and the like. A wireless device can also communicate with other wireless devices either within or outside the same coverage area of a base station via device-to-device (D 2 D) communications. 
     The communication links  114 - 1  through  114 - 11  (also referred to individually as “communication link  114 ” or collectively as “communication links  114 ”) shown in system  100  include uplink (UL) transmissions from a wireless device  104  to a base station  102 , and/or downlink (DL) transmissions, from a base station  102  to a wireless device  104 . The downlink transmissions can also be called forward link transmissions while the uplink transmissions can also be called reverse link transmissions. Each communication link  114  includes one or more carriers, where each carrier can be a signal composed of multiple sub-carriers (e.g., waveform signals of different frequencies) modulated according to the various radio technologies. Each modulated signal can be sent on a different sub-carrier and carry control information (e.g., reference signals, control channels), overhead information, user data, etc. The communication links  114  can transmit bidirectional communications using FDD (e.g., using paired spectrum resources) or TDD operation (e.g., using unpaired spectrum resources). In some implementations, the communication links  114  include LTE and/or mmW communication links. 
     In some implementations of the system  100 , the base stations  102  and/or the wireless devices  104  include multiple antennas for employing antenna diversity schemes to improve communication quality and reliability between base stations  102  and wireless devices  104 . Additionally or alternatively, the base stations  102  and/or the wireless devices  104  can employ multiple-input, multiple-output (MIMO) techniques that can take advantage of multi-path environments to transmit multiple spatial layers carrying the same or different coded data. 
     Internet Protocol (IP) Multimedia Subsystem 
       FIG.  2    illustrates an example functional architecture of an IMS  200 . Some of the network functions provided by the IMS include: 
     1. Call Session Control Function (CSCF)  210 . 
     1a. A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal. 
     1b. An Interrogating-CSCF (I-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain so that remote servers can find it, and use it as a forwarding point (e.g., registering) for SIP packets to this domain. 
     1c. A Serving-CSCF (S-CSCF) is the central node of the signaling plane. 
     2. A Breakout Gateway Control Function (BGCF)  220 . 
     The BGCF is a SIP proxy that processes requests for routing from a Serving-Call Session Control Function (S-CSCF) when the S-CSCF has determined that the session cannot be routed using DNS or Electronic Numbering (ENUM) lookups (ENUM)/DNS. It includes routing functionality based on telephone numbers. The BGCF is used to select the network where the public switched telephone network (PSTN) connection is going to be made. 
     3. An Interconnect Session Border Controller (ISBC)  230 . 
     The ISBC (also abbreviated I-SBC) manages in-coming and out-going traffic from and to the IMS domain and protects the IMS from external attack. The ISBC addresses the boundary requirements at the point that service provider networks interconnect and exchange inbound and outbound SIP sessions. The ISBC integrates three IMS functional elements: Interconnect Border Control Function (IBCF), Inter-Working Function (IWF), and Transition Gateway (TrGW). 
     For a call or message originated from the IMS core, a User Equipment (UE) routes the call or message to CSCF, where the CSCF sends a query to the ENUM/DNS database. If the query to ENUM/DNS database is not successful (e.g., the ENUM/DNS database is not provisioned with the extension information, and/or the destination address is not defined), the SIP call or message is routed to BGCF so that routing can be performed based on telephone numbers. 
     BGCF Number Modification 
     The disclosed technology improves upon prior BGCFs by enabling the BGCF to perform number modification in the IMS domain instead of routing the IMS calls to a legacy mobile switching center (MSC) for the MSC to perform any required translations. The disclosed BGCF manipulates the SIP uniform resource identifier (URI) by deleting, adding, or modifying digits in the SIP INVITE URI. In contrast, prior BGCFs (e.g., BGCF  220  in  FIG.  2   ) route all SIP INVITES with, for example, the same dialed area code and prefix (or the same telephone number), to the same destination, e.g., to the same MSC. The MSC then performs the number translation to route to the appropriate destination. References to BGCF throughout this description can refer to the functions performed by the BGCF or to the network node or element or hardware implementing the functions performed by the BGCF. 
     There are several instances where, although two users in two different geographic locations dial the same telephone number, they intend to be connected to local services specific to the geographic area they are calling from. For example, two users in different states dialing a toll-free (e.g.,  1 - 800 ) number to a nation-wide store might desire to obtain information pertinent to the stores in their state and thus may desire to connect to a target destination associated with their calling location. The BGCF is designed to manage number analysis breakout and is the nearest network element that is connected to the ISBC and then to peering partners. Consequently, by performing number modification at the BGCF, the disclosed technology improves (e.g., reduces) network latency (by removing one hop), and also allows the outdated and expensive MSCs to be retired, among other benefits. 
       FIG.  3    is a flowchart that illustrates a process  300  for generating a SIP INVITE URI based on a P-Access-Network-Info (PANI) header information. At block  310 , the BGCF determines or analyzes PANI header information. The PANI header is a  3 GPP-specific header that indicates to the IMS network, inter-alia, over which access technology and cell the UE is attached to in IMS. 
     For example, an E-UTRAN (LTE) PANI header, includes an access-type field that can be “3GPP-E-UTRAN-FDD” for FDD access or “3GPP-E-UTRAN-TDD” for TDD access. The PANI header also includes a cell identifier parameter, “utran-cell-id-3gpp,” which is coded as a text string concatenation of the mobile country code (MCC), mobile network code (MNC), tracking area code (TAC), and the cell-identity value obtained from lower layers of the UE. In 5G new radio (NR), the primary-access-network-info (PANI) contains access parameters as “3GPP-NR-FDD” or “3GPP-NR-TDD,” with cell identifier parameters MCC, MNC, TAC, and NR Cell Identity (NCI). 
     Each LTE/NR cell site and each tracking area in the LTE/NR radio access networks (RAN) has a unique PANI header. 
     For example, turning to  FIG.  4    which is a block diagram that illustrates an example toll free number modification, two SIP INVITES are shown: SIP INVITE 1  440   a , and SIP INVITE 2  450   a . The two SIP INVITES have two different PANI headers:
         SIP INVITE #1 PANI=3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=3102602C0909c4701   SIP INVITE #2 PANI=3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=31026059078b5623       

     Turning back to  FIG.  3   , at block  320 , the BGCF determines a Major Trading Area (MTA) value based on the PANI header information. 
     For example, the BGCF  420  in  FIG.  4    can include a TAC-MTA lookup table  422  that associates different TAC values to different MTA values. For example, SIP INVITE #1 PANI header includes a TAC value with hexadecimal digits “2C” which the TAC-MTA table  422  associates with an MTA value of 24 corresponding to Seattle; SIP INVITE #2 PANI header includes a TAC value with hexadecimal digits “59” which the TAC-MTA table  422  associates with an MTA value of 1 corresponding to New Jersey. 
     In some implementations, BGCF  420  includes different lookup tables, databases, or other stored pre-determined or dynamically generated translation functions to translate PANI information indicative of geographic location (e.g., TAC and cell-id information) to an MTA value or other value indicative of geographic location of the calling UE, and then to number prefixes to be used in the modified SIP request URI (R-URI). For example, determining the number prefix associated with the geographic location information to be used in the modified R-URI can include first querying a first database (e.g., TAC-MTA table  422 ) that associates different TAC values to corresponding MTA values to determine an MTA value corresponding to the PANI header. Once the MTA value is obtained, the BGCF can then query a second database that associates different MTA values to corresponding number prefixes (e.g., POST RTE Table  424 ) to determine the number prefix associated with the geographic location information to use in the modified R-URI. 
     At block  330 , the BGCF determines a number prefix based on and associated with the MTA value. 
     For example, for the SIP INVITE #1 corresponding to a Seattle MTA of 24, the BGCF  420  determines a number prefix “11253353” and for the SIP INVITE #2 corresponding to the New Jersey MTA of 1, the BGCF determines a number prefix “11631220”. 
     At block  340 , the BGCF generates or modifies a SIP INVITE uniform resource identifier (URI) based on the number prefix. 
     For example, for the SIP INVITE #1, the BGCF  420  generates a SIP INVITE URI  440   b  that includes the number prefix “11253353” and for the SIP INVITE #2, the BGCF generates a SIP INVITE URI  450   b  that includes the number prefix 11631220”. 
     In some implementations, the SIP INVITE URIs modified by the BGCF  420  as described above are sent to the ISBC  230  or to other BGCFs in other networks (not shown in  FIG.  4   ) directly without any intermediate network nodes or additional processing of or modification to the modified SIP URI (e.g., without further adding, deleting, or modifying digits in the SIP INVITE URIs). Consequently, no additional third-party hardware between the BGCF and the ISBC (e.g., the legacy ISBC  230  of  FIG.  2   ) is required for SIP URI processing. Such additional hardware, if needed, can add to the cost of the call routing implementation, can add to the network latency, and can add additional failure nodes or points, among other disadvantages. 
       FIG.  4    shows an extracted data view  424  of statically generated data from a Post Router (POST RTE) Table or database (DB). The data view  424  shows how the BGCF  420  modifies the SIP INVITE URIs for SIP INVITE #1 and SIP INVITE #2 to provide SIP INVITE URIs to the ISBC  230  that include location-based prefixes (e.g., BGCF  420  prepends a location-based number prefix to an initial/un-modified SIP URI that it receives from the CSCF). 
     For example, when users at two different geographic locations, one in Seattle (SIP INVITE #1,  440   a ) and another in New Jersey (SIP INVITE #2,  450   a ), both dial the same toll-free number “+18443355909,” the BGCF  420  adds or prepends a prefix “11253353” to the SIP URI corresponding to the Seattle user and prepends a prefix “11631220” to the SIP URI corresponding to the New Jersey user. The ISBC  230  sends the call to a toll-free provider, where the toll-free provider routes to the correct destination based on the prefixes. Without the number modification in the disclosed technology, unmodified SIP URIs from a legacy BGCF would have the prefix +1844 and would be routed to the same MSC. The MSC would then perform the translation to route it to the correct geographic-based destination. The disclosed technology thereby allows the network operator to eliminate the use of legacy MSCs for number modification. 
     Update to the radio access network (RAN), e.g., adding or removing cells or tracking areas, can require updates to the translation tables in the BGCF  420  (e.g., updates to the TAC-MTA Table  422  and/or to the POST RTE Table  424 ). 
       FIG.  5    is a flow diagram  500  that illustrates number modification for an IMS-based toll-free telephone call. For example, a user equipment (UE)  510  initiates an IMS call and sends a SIP INVITE request  514  to a Session Border Gateway (SBG). The SIP INVITE  514  includes a request-URI (R-URI) of “sip:18xx” (“18xx” corresponds to a toll free 1-800, 1-833, 1-844, etc., number). 
     The SIP INVITE request  514  includes a PANI header  512  with a utran-cell-id-3gpp value that is based on the location of the cell that the UE is attached to (e.g., the uttran-cell-id-3gpp corresponds to an MTA value as described above in relation to  FIGS.  3  and  4   ). For example, as shown in  FIG.  5   , the SIP INVITE has an example PANI header of “3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=310260591e0ac4303” that includes a TAC hex value of “59” which corresponds to a New Jersey MTA. 
     The SBG forwards the SIP INVITE to the S-CSCF (SCSCF) which forwards it to a telephony application server (TAS). Based on a response from the TAS, the SCSCF sends a SIP INVITE  520  to a BGCF  530 . The SIP INVITE  520  includes an R-URI that includes the toll-free number dialed by the UE  510 . 
     The BGCF  530  modifies the R-URI and sends a SIP INVITE  532  to an ISBC  540  (or to other BGCFs in other networks). The SIP INVITE  532  includes a modified R-URI. For example, the BGCF  530  adds steering digits to the dialed toll-free number based on the location of the caller. The caller location is derived from the PANI header (e.g., from the TAC and/or cell-id information in the PANI). As shown in  FIG.  5   , the BGCF  530  can add local numbering plan area (NPA) (e.g., area code) and central office code (NXX) (e.g., prefix) to the R-URI where the NPA/NXX depends on the PANI information  512 . Consequently, as shown in the example of  FIG.  5   , if the user dialed a 18xx toll-free number (e.g., 1-833-454-7878) from a cell site in New Jersey, the BGCF can prepend “11631220” to the 18xx R-URI resulting in a modified R-URI of “sip:+1163122018334547878.” If a different user dialed the same 18xx toll-free number from a different location (e.g., from a different state), the BGCF  530  can prepend a different prefix to the 18xx R-URI such that the two users, although dialing the same toll-free number, will be routed to different destinations. 
       FIG.  6    is a flow diagram  600  that illustrates number modification for an IMS-based directory-assistance telephone call. In the example of  FIG.  6   , the UE  510  initiates an IMS directory-assistance call and sends a SIP INVITE request  614  to the SBG. The SIP INVITE  614  includes an R-URI of “sip:411” corresponding to a dialed directory assistance number of 411. 
     The SBG forwards the SIP INVITE to the SCSCF which forwards it to the TAS which translate the 411 number to, for example, a local or national directory assistance 10-digit code (e.g., +1-425-555-1212). 
     The SCSCF sends a SIP INVITE  620  with an R-URI including the 10-digit directory assistance number (e.g., “sip:+14255551212”) to the BGCF  530 . 
     In some implementations, the BGCF  530  strips out the 10-digit directory-assistance number that can be used by all callers dialing 411 regardless of where the callers are located, and replaces it with a 10-digit directory-assistance number that corresponds to the geographic location the caller is calling from. For example, the BGCF  530  can determine from the PANI header  512  TAC hex value that the directory assistance caller is calling from New Jersey. The BGCF  530  can then replace the 10-digit code in the R-URI +14255551212 with a 10-digit code (e.g., “+16312200411”) that is specific to New Jersey. By stripping digits from the initial URI and adding local NPA/NXX information based on the PANI, the BGCF generated/modified R-URI can be used to route the SIP request to the appropriate destination without relying on MSC translation (or without relying on translation by any node or function between the BGCF  530  and ISBC  540 ). 
     In some implementations, the BGCF replaces one or more digits of the initial SIP URI received by the BGCF  530  with one or more digits of the number prefix associated with the PANI header  512 . 
     It will be appreciated that although the example implementation above has used the  411  directory assistance number, other special short-code numbers are equally applicable such as 811 (national call-before-you-dig phone number), 611 (to report a problem with telephone service), other N11 special services numbers, etc. 
     In one example implementation, for an E911 call coming from an emergency CSCF (E-CSCF), if the SIP R-URI includes steering digits in front of the Emergency Service Routing Number (ESRN), the BGF  530  can manipulate the R-URI to remove the steering digits before sending the SIP INVITE to the ISBC  540 . 
     In another example implementation, the BGCF  530  can be used for SIP routing testing. For example, the TAS can include steering digits in front of the dialed number. When the BGCF  530  receives the SIP INVITE, it can alter the SIP URI by deleting the steering digits and routing the call to the desired SIP route. 
     In another example implementation, the TAS can include steering digits in front of the Mobile Subscriber Roaming Number (MSRN). When the BGCF  530  receives the SIP INVITE, it can alter the SIP URI by deleting the steering digits and routing the call to the desired SIP route. 
     In another example implementations, if a user dials a poison control center number, the BGCF  530  can add steering digit(s) based on the location of the caller so that once the call reaches the network operator, the operator can determine that the call is coming from the correct geographic area. 
     Computer System 
       FIG.  7    is a block diagram that illustrates an example of a computer system  700  in which at least some operations described herein can be implemented. As shown, the computer system  700  can include: one or more processors  702 , main memory  706 , non-volatile memory  710 , a network interface device  712 , video display device  718 , an input/output device  720 , a control device  722  (e.g., keyboard and pointing device), a drive unit  724  that includes a storage medium  726 , and a signal generation device  730  that are communicatively connected to a bus  716 . The bus  716  represents one or more physical buses and/or point-to-point connections that are connected by appropriate bridges, adapters, or controllers. Various common components (e.g., cache memory) are omitted from  FIG.  7    for brevity. Instead, the computer system  700  is intended to illustrate a hardware device on which components illustrated or described relative to the examples of the figures and any other components described in this specification can be implemented. 
     The computer system  700  can take any suitable physical form. For example, the computing system  700  can share a similar architecture as that of a server computer, personal computer (PC), tablet computer, mobile telephone, game console, music player, wearable electronic device, network-connected (“smart”) device (e.g., a television or home assistant device), AR/VR systems (e.g., head-mounted display), or any electronic device capable of executing a set of instructions that specify action(s) to be taken by the computing system  700 . In some implementation, the computer system  700  can be an embedded computer system, a system-on-chip (SOC), a single-board computer system (SBC) or a distributed system such as a mesh of computer systems or include one or more cloud components in one or more networks. Where appropriate, one or more computer systems  700  can perform operations in real-time, near real-time, or in batch mode. 
     The network interface device  712  enables the computing system  700  to mediate data in a network  714  with an entity that is external to the computing system  700  through any communication protocol supported by the computing system  700  and the external entity. Examples of the network interface device  712  include a network adaptor card, a wireless network interface card, a router, an access point, a wireless router, a switch, a multilayer switch, a protocol converter, a gateway, a bridge, bridge router, a hub, a digital media receiver, and/or a repeater, as well as all wireless elements noted herein. 
     The memory (e.g., main memory  706 , non-volatile memory  710 , machine-readable medium  726 ) can be local, remote, or distributed. Although shown as a single medium, the machine-readable medium  726  can include multiple media (e.g., a centralized/distributed database and/or associated caches and servers) that store one or more sets of instructions  728 . The machine-readable (storage) medium  726  can include any medium that is capable of storing, encoding, or carrying a set of instructions for execution by the computing system  700 . The machine-readable medium  726  can be non-transitory or comprise a non-transitory device. In this context, a non-transitory storage medium can include a device that is tangible, meaning that the device has a concrete physical form, although the device can change its physical state. Thus, for example, non-transitory refers to a device remaining tangible despite this change in state. 
     Although implementations have been described in the context of fully functioning computing devices, the various examples are capable of being distributed as a program product in a variety of forms. Examples of machine-readable storage media, machine-readable media, or computer-readable media include recordable-type media such as volatile and non-volatile memory devices  710 , removable flash memory, hard disk drives, optical disks, and transmission-type media such as digital and analog communication links. 
     In general, the routines executed to implement examples herein can be implemented as part of an operating system or a specific application, component, program, object, module, or sequence of instructions (collectively referred to as “computer programs”). The computer programs typically comprise one or more instructions (e.g., instructions  704 ,  708 ,  728 ) set at various times in various memory and storage devices in computing device(s). When read and executed by the processor  702 , the instruction(s) cause the computing system  700  to perform operations to execute elements involving the various aspects of the disclosure. 
     Remarks 
     The terms “example”, “embodiment” and “implementation” are used interchangeably. For example, reference to “one example” or “an example” in the disclosure can be, but not necessarily are, references to the same implementation; and, such references mean at least one of the implementations. The appearances of the phrase “in one example” are not necessarily all referring to the same example, nor are separate or alternative examples mutually exclusive of other examples. A feature, structure, or characteristic described in connection with an example can be included in another example of the disclosure. Moreover, various features are described which can be exhibited by some examples and not by others. Similarly, various requirements are described which can be requirements for some examples but no other examples. 
     The terminology used herein should be interpreted in its broadest reasonable manner, even though it is being used in conjunction with certain specific examples of the invention. The terms used in the disclosure generally have their ordinary meanings in the relevant technical art, within the context of the disclosure, and in the specific context where each term is used. A recital of alternative language or synonyms does not exclude the use of other synonyms. Special significance should not be placed upon whether or not a term is elaborated or discussed herein. The use of highlighting has no influence on the scope and meaning of a term. Further, it will be appreciated that the same thing can be said in more than one way. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import can refer to this application as a whole and not to any particular portions of this application. Where context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. The term “module” refers broadly to software components, firmware components, and/or hardware components. 
     While specific examples of technology are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative implementations can perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or sub-combinations. Each of these processes or blocks can be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks can instead be performed or implemented in parallel, or can be performed at different times. Further, any specific numbers noted herein are only examples such that alternative implementations can employ differing values or ranges. 
     Details of the disclosed implementations can vary considerably in specific implementations while still being encompassed by the disclosed teachings. As noted above, particular terminology used when describing features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific examples disclosed herein, unless the above Detailed Description explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the invention under the claims. Some alternative implementations can include additional elements to those implementations described above or include fewer elements. 
     Any patents and applications and other references noted above, and the assignee&#39;s concurrently filed U.S. patent application Ser. No. ______ entitled CROSS-DOMAIN ROUTING BASED ON SESSION INITIATION PROTOCOL INFORMATION, are incorporated herein by reference in their entireties, except for any subject matter disclaimers or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls. Aspects of the invention can be modified to employ the systems, functions, and concepts of the various references described above to provide yet further implementations of the invention. 
     To reduce the number of claims, certain implementations are presented below in certain claim forms, but the applicant contemplates various aspects of an invention in other forms. For example, aspects of a claim can be recited in a means-plus-function form or in other forms, such as being embodied in a computer-readable medium. A claim intended to be interpreted as a mean-plus-function claim will use the words “means for.” However, the use of the term “for” in any other context is not intended to invoke a similar interpretation. The applicant reserves the right to pursue such additional claim forms in either this application or in a continuing application.