Installed but haven't used the plugin as yet...just looking it over. Two things that come to mind is that if a person already had a list of numbers it would be great if you could add/edit/delete from within the admin interface. This would also be particularly useful if you had a closed (no widget needed) list of folks that needed notifying and didn't want the "public" from adding their number via the widget.
The other thing that might be cool would be the ability to have groups. I can see that having the ability to send to specific sets of people might be useful instead a full list. It might be more even functional with this tweak.
Just thinkin' out loud.