Version 7.2 315 Mediant 800B Gateway & E-SBC
User's Manual 15. Services
15.8.3.3 PSAP Callback to Skype for Business Clients 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 in the ELIN table. (The Count is not affected).
The PSAP callback can be done only within a user-defined period (see 'Configuring the E9-1-1
Callback Timeout' on page 317), 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".
Table 15-27: Choosing Caller of ELIN
ELIN Time Call From
4257275678 11:00 4258359333
4257275678 11:01 4258359444
4257275678 11:03
4258359555
15.8.3.4 Selecting ELIN for Multiple Calls within Same ERL
The device supports the receipt of up to five ELIN numbers in the XML message of each
incoming SIP INVITE message. As discussed in the preceding sections, the device sends the
ELIN number as the E9-1-1 calling number to the emergency service provider. If the XML
message contains more than one ELIN number, the device chooses the ELIN according to the
following logic:
If the first ELIN in the list is not being used by other active calls, it chooses this ELIN.
If the first ELIN in the list is being used by another active call, the device skips to the next
ELIN in the list, and so on until it finds an ELIN that is not being used and sends this ELIN.
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