Yes this could be done with a pretty simple script file running on asterisk or freeswitch. You would need a VoIP origination provider (or a few)...a list of spoofed numbers which could be any numbers.
Trouble is that this would several felonies committed. Spoofing a caller Id is illegal, as well as interfering with communication systems...could get in big trouble.
A couple offshore linux vps servers, a few prepaid sip Trunk accounts, and some balls to pull the trigger could easily gum up a small/med sized company's phone lines for a day or two before anyone could shut it down.
http://en.wikipedia.org/wiki/Swatting
Yes something like this happened to me years ago. It was similar but not the same. This was in the early 90s when people used pagers- what happened was someone figured out how to send a message to 1000 pagers at once. So I get a page- says some number -911-911-911 so well I make sure to call quick. Turns out it was a fire station a couple of towns over. Guy that answered said he had gotten 2-300 calls that day.snip snip
snip snipCode:http://en.wikipedia.org/wiki/Swatting
i remember someone (professinal20?) said here that he is capable of sending a few thousand SMS to a phone number in a very short timeframe, this little operation basically paralize the phone at least for a while
You need to look into learning Voip specifically how to run asterisk.
Add in some throw away plastic and a voip account and you could have an asterisk box call a phone number 20,000 times for under $50.00 as an Added bonus you could have their caller ID show up as their local dildo shop.
Also if you learn tons about Voip there are ways to attack Voip systems that run phone systems directly and take down the system.
Enough to start googleing?