What it does

Public Safety Answering Point (PSAP) callback is a service that helps emergency services call back a subscriber after an emergency call has ended. On the Rhino VoLTE TAS, the PSAP callback service ensures that none of a subscriber’s terminating MMTel TAS services (for example, Incoming Call Barring) hinder the callback attempt.

PSAP callback detection

There are two mechanisms available for detecting when a PSAP callback could be occurring:

  • direct notification from the Emergency Call Session Control Function (E-CSCF), or

  • analysis of the Priority header on incoming initial INVITE requests.

Both mechanisms can be used simultaneously. If either mechanism indicates a PSAP callback attempt, then the call will be handled as such.

E-CSCF notification

The Rhino VoLTE TAS can be configured to receive SIP MESSAGE requests from the E-CSCF when a subscriber calls an emergency number. If this behavior is enabled, upon receiving a such a request, the TAS makes a record of the subscriber’s identity that expires after a period of time determined by the expiry-time-seconds configuration field. Until the record expires, all incoming calls to the subscriber are handled as if they are a PSAP callback attempt and many MMTel services will be disabled.

Depending on network requirements, the SIP MESSAGE from the E-CSCF can either be allowed to continue on to its next destination, or the Rhino VoLTE TAS can stop the MESSAGE and generate a final response to the E-CSCF.

Priority header analysis

The Rhino VoLTE TAS can be configured to check for the presence of a Priority header on all incoming SIP initial INVITE requests. If the header is found with the value psap-callback, that call is handled as a PSAP callback attempt and many MMTel services will be disabled.

Interactions with other services

MMTel terminating services in general

The PSAP callback service’s purpose is to bypass any terminating MMTel TAS services that could potentially prevent emergency services from calling back a subscriber.

When a call is handled as a PSAP callback, the following terminating MMTel services are disabled:

  • Announcements

  • All types of call barring

  • Communication diversion

  • Voicemail forwarding

  • Explicit communication transfer

  • Communication hold

The following terminating MMTel services remain active for PSAP callbacks:

  • Session transfer to own device

  • Flexible alerting

  • Privacy (OIP and TIR)

  • Communication waiting

Online charging

Online charging is handled as a special case. The Rhino VoLTE TAS continues to communicate with the online charging server as normal for the duration of the call, but if the subscriber requires credit to receive the call and it cannot be allocated, the call is still allowed to continue.

Companion device

Certain functions of the companion device service that could interfere with emergency calls are disabled when an emergency call is detected:

  • When a companion device initiates an emergency call, its undisclosed identity will not be hidden.

  • When a PSAP callback attempt is made to the undisclosed identity, the companion device service will not block the call.

Configuration

The example for sentinel-volte-gsm-config.yaml and example for sentinel-volte-cdma-config.yaml show example configuration relevant to PSAP callback in the sentinel-volte/mmtel/psap-callback section.

What you need

  • ❏ Whether or not to use the Priority header to detect PSAP callback attempts.

  • ❏ Whether or not to use the notifications from the E-CSCF to detect PSAP callback attempts.

If using notifications from the E-CSCF:

  • ❏ How long after receiving a notification from the E-CSCF to handle incoming calls for a subscriber as PSAP callbacks.

  • ❏ Whether or not to forward the notification onwards after processing it.

Setting up PSAP callback

I want to…​

Use the Priority header to detect PSAP callbacks

In the psap-callback section, set use-priority-header to true:

            use-priority-header: true

Related section: PSAP callback detection

Disable use of the Priority header to detect PSAP callbacks

In the psap-callback section, set use-priority-header to false:

            use-priority-header: false

Related section: PSAP callback detection

Use notifications from the E-CSCF to detect PSAP callbacks

In the psap-callback section, add a sip-message-options section:

            sip-message-options:
                expiry-time-seconds: 86400
                terminate-message: true

  • The expiry-time-seconds field must be set to the period of time in seconds that incoming calls for a subscriber should be treated as PSAP callback attempts.

  • The terminate-message field must be set to indicate whether the notification message should be answered by the Rhino VoLTE TAS or forwarded on.

Related section: PSAP callback detection

Disable use of notifications from the E-CSCF to detect PSAP callbacks

In the psap-callback section, remove or comment out the sip-message-options section:

#            sip-message-options:
#                expiry-time-seconds: 86400
#                terminate-message: true

Related section: PSAP callback detection

Change how long after receiving an E-CSCF notification that incoming calls for a subscriber should be handled as PSAP callbacks

In the sip-message-options section, set the expiry-time-seconds to the desired time period in seconds:

                expiry-time-seconds: 10000

Related section: E-CSCF notification

Forward E-CSCF notifications onwards to their destination after processing them

In the sip-message-options section, set the terminate-message to false.

                terminate-message: false

Related section: E-CSCF notification

Not forward E-CSCF notifications onwards and instead send a final response for them

In the sip-message-options section, set the terminate-message to true.

                terminate-message: true

Related section: E-CSCF notification

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Rhino VoLTE TAS Version 4.1