google these:
group phone call
blast message
automated mass phone call
group message service

our school uses a system like this to announce snow days

and the sherriff recently sent out a call when there was a missing child in the area

when you gather the phone numbers of people make sure you
have a number that is connected to an answering machine or
voice mail...you might even include a couple numbers for some
folks...such as cell and land line...if you want to increase
the chance of your message being rec;d.

the message below is an ad for www.freeconferencecall.com
there might be something there..good luck and let us know
what you find!