I have had good success when a "single-speaker" option is sufficient by using the Benbria Blazepoint talk-back speakers found on eBay for $37.50 + $12.00 shipping.
These speakers are VoIP based and auto-answer when the extension they're assigned in the iPBX is called. Although they're limited in function These speakers are now part of the Mitel Mass Notification product line - so without the Mitel front end MMN server (I think that it's software based) some advanced functions are not available, like 'canned' announcements or 'emergency' messages delivered to selected speaker groups. But, as a simple overhead paging speaker is concerned, these talk-back speakers work very well!
The speakers default to DHCP after being reset. Setup is SSH based, and logging into the default speaker account (bpadmin, password admin), the configuration is entirely command-line based - no GUI. After logging in, use "tab" or type "?", the available commands are provided. To view the available options, enter one of the listed options, followed by a "tab" or "?" to view the follow-on options to configure the speaker to work with an installed iPBX system. There are multiple layers of commands, but it's not difficult to figure out.
These speakers from eBay were removed from service at a Texas university when they upgraded their paging system/installation, are very clean and show no cosmetic signs of wear.
/Pete./