PSAP Callback for Dropped E9-1-1 Calls
As the E9-1-1 service automatically provides all the contact information of the E9-1-1 caller to the PSAP, the PSAP operator can call back the E9-1-1 caller. This is especially useful in cases where the caller disconnects prematurely. However, as the Enterprise sends ELINs to the PSAP for E9-1-1 calls, a callback can only reach the original E9-1-1 caller using the device to translate the ELIN number back into the E9-1-1 caller's extension number.
In the ELIN table of the device, the temporarily stored From header value of the SIP INVITE message originally received from the E9-1-1 caller is used for PSAP callback. When the PSAP makes a callback to the E9-1-1 caller, the device translates the called number (i.e., ELIN) received from the PSAP to the corresponding E9-1-1 caller's extension number as matched in the ELIN table.
The handling of PSAP callbacks by the device is as follows:
| 1. | When the device receives a call from the emergency service provider, it searches the ELIN table for an ELIN that corresponds to the received called party number in the incoming message. |
| 2. | If a match is found in the ELIN table, it routes the call to the Mediation Sever by sending a SIP INVITE, where the values of the To and Request-URI are taken from the value of the original From header that is stored in the ELIN table (in the Call From column). |
| 3. | The device updates the 'Time' field in the ELIN table (the 'Count' field is not affected). |
The PSAP callback can be done only within a user-defined period (see Configuring the E9-1-1 Callback Timeout), started from after the original E9-1-1 call, established with the PSAP is terminated. After this time expires, the table entry with its ELIN is disregarded and no longer used (for PSAP callback). Therefore, table entries of only the most recently terminated E9-1-1 callers are considered in the ELIN table. If the PSAP callback is done after this timeout expires, the device is unable to route the call to the E9-1-1 caller and instead, either sends it as a regular call or most likely, rejects it if there are no matching routing rules. However, if another E9-1-1 caller has subsequently been processed with the same ELIN number, the PSAP callback is routed to this new E9-1-1 caller.
In scenarios where the same ELIN number is used by multiple E9-1-1 callers, upon receipt of a PSAP callback, the device sends the call to the most recent E9-1-1 caller. For example, if the ELIN number "4257275678" is being used by three E9-1-1 callers, as shown in the table below, then when a PSAP callback is received, the device sends it to the E9-1-1 caller with phone number "4258359555".
Choosing Caller of ELIN
|
ELIN |
Time |
Call From |
|---|---|---|
|
4257275678 |
11:00 |
4258359333 |
|
4257275678 |
11:01 |
4258359444 |
|
4257275678 |
11:03 |
4258359555 |