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Call Paths:

Subscribers registered on the Ribbon AS from Bria, Poly, Calix, or Adtran clients are processed by the ERS (Emergency Response System) of the AS for location information when the make an emergency call.  These locations are populated in the AS and associated with the user in the database.  Each Location is treated as an ERL (Emergency Response Location) which has an associated ELIN (Emergency Location Identification Number).  This ELIN is substituted for the Calling Party Number for the call in the SIP Invite and the call is then sent over a SIP Trunk to our E911 service provider Intrado.  The Intrado ERS processes the incoming call and matches the ELIN with provisioned Location information and routes the call to the appropriate PSAP (Public Safety Answering Point) which is usually UTPD (University of Texas Police Department). In the event that a call is processed by the Intrado ERS service that does not match a provisioned ELIN, then the call is routed to the Intrado Call Response Center where it is answered and possibly forwarded on to a PSAP.  In addition to registered clients, E911 calls from MS Teams Phone Users are routed to the Intrado ERS, and at this time route to the Call Response Center.  Microsoft and Intrado are working to dynamically provision the location information for these calls.  The SIP Trunk connectivity to the Intrado ERS is HA/Geo redundant, with gateway VPN devices located at NAP-M, and NOC-B data centers.  Each gateway can route calls to the two Intrado ERS data centers located in New York or Los Angeles.   In the event that this primary E911 SIP Trunk were to fail, calls will be automatically sent over the main AT&T IPFlex SIP Trunks used for general PSTN Calling.  These trunks are highly resilient, but in the event of a failure of these trunks also, E911 calls will then be routed over 2 geographically redundant ISDN Primary rate trunk spans. 

Pending updated Call Flow Diagram: (coming soon) 


When the Ribbon AS ERS processes a call and sends it to the Intrado ERS, it also generates an XMPP message with the complete provisioned data available for the Calling Party Number, and for the ELIN to clients located at UTPD.  This message possibly contains additional location details not kept in the provisioned information in the Intrado ERS.   The Intrado ERS when processing a call emails all the call and location information to UTPD and Voice Services personnel.  UTPD personnel also have client that allows them to do Mysoft/Pinnacle database lookups by entering the Calling Party Number.

Initial Provisioning:

Fixed VoIP and Analog service for a number are provisioned with an AS Location when deployed that has an associated ERL/ELIN.  This ERL/ELIN information is entered into the billing system, and routine synchronization between the AS and the Mysoft/Pinnacle database is maintained.  Discrepancies generate an update file which is then propagated to the Intrado ERS.  One unresolved wrinkle here is that UT sends updates to two Intrado ERS instances.   Since most of UT’s telephone numbers are classified as traditional E911, when calls are routed over the backup SIP Trunk they ingress into the E911 system through the legacy gateway system, which accesses a different Intrado ERS instance.  This backup instance is sent the same synchronization file as the primary when updates occur.  Efforts are underway to collapse these two instances to a single database. 

Provisioning changes due to Phone moves:

When a Polycom instrument is moved its initial network port to cable pair/jack/building/room mapping needs to be updated.  A weekly scan of network ports for relocated instrument MAC addresses is processed, and new location information for the line 1 subscriber configured on the device (typically a polycom phone) is updated within the Ribbon AS.   However there is currently about a 20% error rate due to incorrect network port to network jack mapping.  This mapping information discrepancy needs to be addressed.  For a detailed description of the UT process to maintain location mapping for fixed VoIP devices (Polycom phones) please see this Wiki Link: 

MS Teams Phone Service and Remote Workers:

The increase in off campus workers using remote clients has increased substantially in response to the Covid-19 pandemic.   The extension of UT Voice Calling to these remote clients has driven the need to be able to dynamically provision E911 Location information sent to the Intrado ERS to determine the correct PSAP to route the call to for off campus responders.   Both MS Teams and Intrado are in the process of modifying their service to accommodate this requirement.  As of 09/22/2021 the UT's Intrado ERS account has been modified and provisioned to accomodate potential calls from users provisioned with MS Teams Phone service.  Microsoft updates to MS Teams, and the necessary configuration changes to generate the dynamic location are still pending as of 09/22/2021.

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