PA announcement software

I was wondering if there was any freeware, or even something cheap, for setting up automated announcements over a PA system. I am in charge at a bus terminal, and currently make announcements every half hour or so over the PA alerting passengers to bus arrivals and departures. I was wondering if there is some software that I can put on my computer to make the announcements for me.

I know that the PA system is tied to the phones somehow because we can dial an extension from the phone and make announcements. We use an IP phone system here, so I was hoping that all it would take is a software that can either connect to the phone system via ethernet, or even connect directly to the PA system via it's IP address, and then at set times make announcements. Also I would like to be able to change the announcements on bad weather days to state that there may be weather related delays. Any help is greatly appreciated.