Just my 2 cents here;

One of my customers just went through this VOIP business. The OEM equipment had its own dialer interface module that originally was an analog old school line. One line per car. When the powers that be switched to this newer VOIP thing, all elevator phones were lost. As I understand it, the IT guy had to get different style phones, Rath I believe, in order to restore phone operation. So, it may not work in all situations.
Land lines dedicated to the elevator phone, set up to dial out to a 24hr answering service is how it's done on the west coast.
Cell phone operation is all but lost from inside the cabin itself almost always. That's ALMOST.

Hydraulic elevators by themselves do not simply drop or lower themselves to the bottom floor in the event of power failure.
There has to be an external means, i.e. ups or back up power source. It needs to be designed, read "engineered" to interface with elevator circuitry to lower a hydraulic elevator, and open its doors in a power outage condition.
VOIP has nothing to do with a lowering system, or its back up power source.

Off the shelf ups systems are just that. Fine for your home computer which draws minimal current to save your data in a power loss.
Each ups device inserted into an elevator control is like I said, engineered to do the job it's intended to to meet state mandated code requirements.

Know what your local codes are which govern what can and cannot be done to your public vertical transportation system.

As mentioned, some things seems easy enough and seem to make sense to accommodate two way communication, but if someone alters that system for whatever reason and there is harm or damage as a result...your had if it's proven you modified any system rendering it out of compliance of local codes or ordinances resulting in injury, harm or whatever.

That's what lawyers are for. And yes, some of us do speak from experience as we have sat in a courtroom answering to a jury being hammered by an attorney who has some "expert witness" (industry expert) looking to see just what mis-steps we or the company has made in order to cook us on behalf of their plaintiff.

OK, so it's more like my 50cents worth...sorry for the long read.