This has nothing to do with callbacks, this is how the emergency call will be routed and what caller ID be presented to provider/911 operator.
For each ERL you can define multiple ELINs, ELINS are the specific DIDs allocated to e911 and are defined in the ALI database that map the address to this number, so each ERL needs unique ELINs, number of ELINs depends on the size of the ERL and how many users are part of it, is if more than one call is made from the same ERL you want to use different one as CER recycles them.
Route Pattern/TP is basically telling CUCM what should it use to route the call out, typically you only need one Route Pattern per location, and by using "calling party modification" the ELIN is used as the caller ID of the call.
913XXXXXXXXX is a CTI Route Point on CUCM that allows callbacks to be redirected to CER.
So here is a high level call flow for outbound emergency call:
1. phone with DN 1000 dials 911
2. CUCM matches Translation pattern in site specific partition and relays the call to 911 CTI RP, or this CTI RP is matched directly
3. The CTI RP is associated with ceruser, and hence registered to with CER
4. CUCM transfer the call to CER
5. CER looks up the details of the caller based on CDP and SNMP locating either the switch port the phone is connected to or subnet (based on config in CER)
6. The switchport or subnet is associated with ERL
7. CER uses the next available ELIN of the ERL, and uses the RP/TP, it routes the call back to CUCM
8. CUCM matches the RP/TP and modifies the calling number of the ELIN
9. Call is send out to PSTN
10. when call arrives to PSAP the ALI database matches the calling number and maps the address of the caller
For the callback, assume the call was dropped and 911 operator dials the ELIN
1. call arrives at CUCM and matches a specific translation pattern that summarizes all of the ELINs
2. The TP prefixes 913 to the called number
3. 913XXXXXXXXX CTI RP is matched, which is registered to cer
4. call is transferred to CER, CER lookup up who was the last caller that dialed 911 using this ELIN (this info is held for up to 3 hours)
5. Call is extnded to the original caller, extension 1000 in this example.
HTH,
Chris
