I'd sometimes put a basic IP phone as with a DSS labeled PAGE. If we were fortunate, the phone would have a headset jack, and setting to allow it to auto-answer. The headset cable was connected to the page amp input. Worse case, I'd solder leads to the speaker terminals and have the phone in a page group.
Did the soldering many times on an Iwatsu systems using a Versa or the other similar one (MKT?). For an VoIP system, using a basic IP phone can be cheaper than an adapter of some sort.
I thought it was great how the Iwatsu systems had relay closures for whatever. We configured the keys on the phones for door openers, and other applications. One to trigger an alarm in that Icon system - forgot the name, (Signals?)...it would send an email alert and call the local emergency dispatch center. We'd label them red, but that didn't always prevent false alarms. That includes by me when changing a faceplate.
