Probably best to avoid using the VOIP terminology in regards to the 21CV service VM is offering at present. It uses VOIP technology but is not a true VOIP connection in the home in its present format. The present setup is designed to use a (modern) conventional landline phone plugged into the back of the VM hub.
VM have gone through every possible combination of responses on here about re-connecting existing phone extension sockets during the phone switchover, ranging from a flat 'no we don't do that' through a 'possibly/maybe' answer all the way to a 'free connection of existing extensions as part of the process'.
This page has recently been edited and re-formatted and
described a free technician visit to connect up existing phone extensions as part of of the switchover.
Note the section which states
You’ll need to book a free technician visit if:
You rely on your landline for accessibility needs or don’t have a mobile to make an emergency call – we’ll provide you with an Emergency Back Up Line so you’re always able to call emergency services.
You’ve got connected devices such as a burglar alarm linked to a control centre, use a telecare device, or have other phones connected to extension sockets. You’ll need to contact your provider and let them know about the switch if you have any of these to make sure they’re compatible with a fibre service.
Your Hub and phone can’t be placed near each other.
so the offer still seems to be available but mentions to it being 'free' have been buried within some very small, small-print.
Suggest waiting for the VM forum team to reply here and advise on what today's VM policy is on this.
Your parents should be able to continue using their existing phone setup but it requires a tech to link the phone socket on the hub to the existing home extension sockets. From past topics on here, this either needs to be booked in well in advance so the tech knows to do it as part of the switchover or it gets booked in as a separate visit once the switchover has been completed.
The idea that vulnerable, older customers should possibly have to fork out for new equipment as part of this process does not sit well with me but that currently seems to be VM's direction of travel as they seem to make little effort to correctly advise on the options when the switchover is actually taking place.