Off hook comes from the idea that the handset of a phone is normally hooked on top of the phone, and when you make or receive call, you lift the phone off the hook. For a speakerphone, pressing the button that makes the speaker come is doing this.
On a simple analogue line, going off hook makes the phone busy.
On a VoIP phone, going off hook may only make one line busy and subsequent attempts to ring the phone will ring the other line . You will then have to switch lines to answer the call.
On all modern phones, the amplitude of the ringing is controlled by the phone. On analogue phones, the phone will follow the on and off times of the ringing, but will not pay any attention to the level of the ringing voltage.
On IP and other digital phones, the phone receives a message to say there is a new call and then internally generates both the ringing tone and how it goes on and off with time, based on that message. There may be phone specific ways of selecting how the phone should ring in that message, but amplitude is not something that will be controlled that way.
Therefore any question about how loudly a twenty first century phone rings, or exactly how a digital phone rings should be addressed to the manufacturer of the phone, not to the supplier of the PABX.