I did a quick look and most of the 9-1-1 centers we support have a minimum of three carriers even the small ones with under one-hundread devices. Remember this is not primary station alerting this is paging over commercial carriers.
Sorry, but I disagree. That is undoubtedly the case for a commercial paging center but not for the needs of a typical 9-1-1 center transmitting only emergency notifications to a set of responders. The necessary functionality already exists in hardware/software in many call centers and requires minimal additional maintenance.
I know you want to sell your service but, in all fairness, anyone looking for responder paging via SMS and/or email owes it to themselves to explore all the options. Doing it themselves may very well be the better option for many.