Ann, We would need some more info as far as your infrastructure such as the input audio device (If any). If you are using a VOIP PA Gateway does it have any analog inputs?? If it does you can use a and IP phone (create new extension) and tap off the internal speaker with a line balancing transformer and go to the analog input of your PA system gateway. The you program that Extension to answer automatically when called (where if someone calls that extension it just beeps twice and you can talk) The you create a "Page Button" as one of the users endpoint function buttons but actually program it as a system speed dial number to that extension you have wired to the gateway. 

We have a IP phone system on all of our schools that have central SIP unit at the High School then go over the lan to all the other school so we have this setup on all other 9 schools. Although we connect to a Analog PA system input the theory of operation will be the same you only need a analog input on the IP speaker system you have.

I would contact George at RI Telephone out of Warwick as they have done several of these type of systems and is very reliable.

Hope that helped

Ralph  

North Providence