Senario:
Got inbound call in the middle of the call,power outage occur is there away to retrieve the number of inbound call?
You can possibly do a couple of things -
If using SIP phones, possibly the number if listed in the call history of the phone.
If using SIP trunks, you might be able to check the provider CDR logs.
It might be in the CDR logs of the PBX, maybe.