Paul,
VOIP providers give you a phone number that any phone can call.
In short it is just telephone service over the internet.
You can use a softphone which can be a computer program or even an app on a smartphone.
You can use dedicated piece of hardware. "VOIP phone" that plugs into ethernet.
Or you can even use a real telephone. To use a real telephone you need an ATA "Analog telephone adaptor".
Vonage is an example of an ATA. They used to sell those boxes in Walmart.
Buying a Vonage box forced you to use Vonage as your VOIP provider unless you were crafty enough
to flash the firmware and unlock it.
Voip can use one of many protocols. One is called SIP but that one is not very secure and is fading away.
Cisco has their own proprietary VOIP protocol but I forget the name of it.
Asterisk has their own protocol that I believe is open source called AIX.
With free open source software like Asterisk you can build your own PBX (private branch exchange)
But in order to connect your PBX to the public phone network you would need a VOIP provider.
The VOIP provider sells you a DID (phone number) and provides an access point. (a VOIP server to connect to)
That is probably more than you wanted to know. lol
-Steve