I had similar experience on 9-11. Only thing that worked was sms. A second advantage to sms is that it is pretty universally accessible. You can send a message to multiple addresses also. I would look for a tool that includes sms, and that itself will bring some restrictions on message length. I would think users would want to self-subscribe as they may be paying for the messages.
You could look at this system, it works really well for me at work. My home county has one also.