Now a days quite a few Telco providers offer SIP Trunking For Cisco Call Managers to replace traditional and expenses PSTN Trunks such as PRIs etc.

Some customers go with SIP Trunking for regular calls, but still rely on dedicated FXO trunks especially for 911 Services. I see a little issue with using FXO trunks for 911 Calls though.

For example, if Customer A has 900 DIDs mapped to SIP Trunk and those DIDs are registered with each IP Phones and that allows incoming calls ringing directly to

each IP Phone. The customer also uses SIP trunk for 911 calls instead of FXO Trunk. In such scenerio if one of the IP Phone user calls 911 and then the call gets dropped, but PSAP will be able to see the whole 10 digit number and if required can call back right away to talk to the user and find out where exactly the user is. The same scenerio could not be achieved if FXO trunk is used since the DID number for FXO line will be different than the IP Phones that are registered to DIDs associated with SIP Trunk. Of course, the user can still call 911 thru FXO, but the PSAP will see a call coming through FXO DID instead of DIDs that are mapped to SIP trunk, and will not be able to call back the user if the call gets dropped.

Is there a work around in a Call Manager such as Translation pattern to map the single FXO DID to all 900 DIDs that are associated with SIP trunk so the PSAP can successfully call the user who initiated a 911 call from an IP Phone. If not, then what would be the best way to provide 911 services for customers who have SIP Trunking instead of Traditional PSTN trunks.

Feedback will be appreciated.