Do you mean you have ported your phone number to become your VM residential landline number?
If so, that won't be a VOIP connection it will just be VM's standard landline service which is presented on the TEL1 socket on the back of the VM hub. The Gigaset N300 seems to be a hybrid phone so you would connect the standard landline connection from the N300 into the TEL1 socket on the VM hub. VM should have provided you with an adapter to convert a BT plug from the phone to RJ11 socket on the hub as per bottom LHS pic in the link below
https://www.virginmedia.com/help/home-phone-line-faults
