I have a remote site with a few users and they register their phones to a central CUCM cluster. I submitted a port request to port the numbers belonging to that site to the central sip trunk and the ITSP rejected that request citing the reason that they don't service that area. I believe the ITSP can still port the numbers but without 911 address information.
So I am thinking about getting a POTs line and connect that to an analog phone in a common area in the office for the purpose of 911 calls but want to make sure if that complies with 911 regulations.
You'll need to ask your legal counsel and the local regulatory authorities for guidance on that.
If you are told that the PBX phones need to provide 911 access, then you would want to add an FXO port, DSPs, and voice software licenses to that site's WAN router (or an additional router to be a dedicated voice gateway). Ground-start analog loops are always preferred over loop-start.
I believe that any working phone should have the capability of dialing 911. I agree with Jonathan's approach which would allow that. You can also ask your service provider if they offer a 911 triaged service, but if they don't port the numbers for that interconnect region, then you will still need to get local 1FLs.
I always recommend to have local 1FLs for dedicated 911 access through through the VoIP infrastructure, even when the PSTN access is centralized using a SIP provider. This provides a lot more survivability functionality than just 911 access.
Thank you Giorgioisalsiri and Jonathan for your valuable suggestions. So it looks like all PBX phones will have to have capability to dial 911 in which case the local voice gateway with fxo and POTs line is one of the best solutions. Thank you.