The current VM offering (21st century voice, 21CV) uses standard phones plugged into the TEL1 socket on the back of the hub. Presumably this will morph into a true VOIP service from VM at some point in the future.
VM supply an adapter (which does nothing more than convert the RJ11 socket on the hub to a standard BT socket). You connect an ordinary phone via the adapter to the hub. Probably simplest to set up using cordless phones with the base station plugged into the hub and cordless satellite phones used around the home as required.