You might be able to use the Call Creation API to pull off what you want, but I can't say for certain. Needs to be detailed out more and tested to make absolutely sure it will work.
I agree with the previous comment that a paging system might be the best solution:
Cyberdata makes good paging products. They have amplifiers that takes a single SIP endpoint and sends it to multiple speakers (like only ringing one phone). Or they have nice ceiling speakers that are SIP endpoints as well.http://www.cyberdata.net/products/voip/ ... oints.html