SIP trunking design

I've done quite a bit of research/reading on SIP trunks (including a great Cisco Press book on the subject), and have a design question.

A customer with 20+ remote sites in smaller areas is deploying a Cisco voice solution, and we initially were hoping to deploy a centralized SIP trunk solution through the main site, and use SIP/SCCP signaling over the WAN to the remotes.  So (in my thinking), only the main site would need to support SIP trunking from the carrier.

The carrier (AT&T) informed me that they couldn't support this, since the remote sites were not in "rate centers", and that they didn't support direct SIP trunking themselves.  Put simply - they claim we can't do centralized SIP trunking unless each/all of the remote sites COULD support SIP trunking from the carriers themselves. The reason given had to do w/ 911 regulations.

Can I get some clarification on this issue?  Is it only possible/practical to realize the benefits of centralized SIP trunking if ALL of the locations are within larger metro areas that could support SIP trunking directly from the carrier?

Thanks again, and reference links are always appreciated.

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lrmooreConnect With a Mentor Commented:
The carrier is correct in that they have to maintain the ani/ali database of phone numbers to physical locations and make sure 911 calls get routed to the proper PSAP. If you have a branch in Boca Raton, and your primay SIP trunk is in New York, there is a problem if someone at the branch calls 911 and NYPD gets dispatched to your main office. If the branch calls 911, AT&T would have to know to route a call from that location to PSAP in Boca, and the ani/ali location information must be accurate. So in essence, it's a long distance call, or outside the rate center, for AT&T to make that call.
Typically we recommend a couple of analog POTs lines at each remote location where we would route some local and all 911 calls from that site. You can have a central SIP trunk in NY with a block of DiD numbers and assign them to branches however you want. But there you run into a situation where long distance charges may apply if the Boca Raton office gives out a NY prefix telephone number to call them from the coffee shop across the street.
cfan73Author Commented:
Thanks for the response, and quick follow-up. We always deploy FXO ports in each remote site voice gateway for a few analog trunks - 1) for SRST survivable and 2) 911.  We configure the voice gateway to send 911 calls out the local trunks.  Having said that:

I threw this past the carrier at question, and he still balked at the design, claiming that the local numbers still had to be "within rate centers" - essentially, the remote sites had to support SIP trunk connections themselves even though we were trying to build a centralized SIP design.

I'm clearly still not seeing this clearly, so any further clarification would be appreciated.

cfan73Author Commented:
Any additional feedback/insight into the above, hopefully?
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