I don't know much about overhead paging but as an alternative you can add a group-page in addition to the overhead paging.
You could do a command: list group-page to see if you have groups already setup.
Then change group-page x
Add in the extensions in the zone you like and assign an extension to dial for paging.
When you dial the extension to page, it will activate the speakerphones of the phones in the group-page list, and announce your page.
If there are more than 32 phones in the zone, you could add in every other extension. Usually group-pages are pretty loud and you don't have to add every single phone in the area.