I didn't think PBX systems usually run off of VOiP, I thought they ran on a traditional landline. I'll admit I don't know much about it. Do people switch to VOiP entirely and never use landlines and cell phone systems?
In VoIP, the PBX has to run on both the traditional landline/trunk and to the IP network. It's what connects the two.
I ran one for a start-up that went from 35 when I joined to 110 employees. We had a hundred phone numbers, but only paid for eight landlines, so only up to 8 calls to outside the office could happen at once.
These days, any business that puts a computer on every employee's desk is going to use a VoIP PBX. It's cheaper and easier to maintain since you have to wire every desk to the network anyway, and you don't want pay to use the telephone network every time one employee calls another.
Even Dunder-Mifflin uses VoIP in The Office. Most likely a paid product placement arrangement, but still. That model of phone is everywhere.
I'd think they'd use a geolocation service to look for the approximate location of the IP address even with VOiP
They don't get to see the caller's IP address. That information is lost when the call went from the VoIP side to the telephone network. They see a number assigned to the PBX administrator. They have to talk to the PBX admin to find out which IP the call originated from.
I'd also think most people would prefer to use a system that makes their location available if they're calling 911, I've only called on my cell phone.
VoIP numbers are required by Federal law to register a physical location, or get an affirmative opt-out from the customer in cases like connecting remotely or with a mobile device. With VoIP the 911 operator gets the caller's address just as fast as a plain landline, and faster than if you used a cell phone. Plus cell phone 911 calls are not as efficiently routed as landline/VoIP 911 calls.
We had three accidental 911 calls in the years I ran a PBX, and the police showed up within 10 minutes every time, routinely having a look around and politely threatening to fine us if it happened again, though they never did.