Legacy paging systems and modern voip systems are usually a nightmare these days. Most legacy paging systems were set up to accommodate the PBXs of the times. Some used trunk interfaces (FXO), some used analog ports (FXS) and some used paging ports like Norstar/NCMs. Almost all modern voip systems can have the FXO or FXS capability with the right hardware but even with that, there is usually some kind of paging module to connect to. Your system probably has that if you're looking at an FXO interface. If I had the choice though, I would use an FXS interface.
The nice part about the VOX-SIP, Cyberdata, Algo etc units is they are sip devices that register directly to the PBX whether premise based or hosted and provide a proper 600 ohm output that is designed to connect directly to legacy amplifiers without the need for any kind of adapter. I've used all of them and they all work well but the one thing I liked about the VOX unit was if you don't want the scheduled announcement option, you don't have to pay for it. If and when you want to add it, you just buy a license to enable it so for basic paging, it is a lot cheaper than the others. It also has a separate input for a music source and also has several notification tones loaded into them (you can add your own if you wish). There is a single output from it that connects to the "tel" connection of the amp and all audio is sent out via this connection. Volume controls for the paging, music and tones is controlled separately via the VOX unit with the master volume being the control on the amp. Very easy to set up and use and a lot more secure.
I went to these because I got tired of dealing with paging systems that were set up by people who didn't know what they were doing and trying to figure how to duplicate their mistakes. This unit (and the others) give you pretty much complete control over paging without all the complications and poorly installed systems.
You can do it with ATAs as long as the paging system is one that can handle them. If you choose FXO, remember that for it to work, the end device must supply the talk battery (48v) for it to work and amplifiers do not do that (hence the paging adapters) and if you use FXS, you will provide the 48V and the amplifier has to be able to handle that as well as the ring voltage (90VAC). so good luck with it.
