the Voip on the hub 3 is mainly for new customers, you won't be switching to that, eventually it will be required but this is probably a ways off, your phone will carry on working as it always has via hard wire back to the cabinet
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
You phones will connect to the same place they are, only if you were told that you would be transferring to the VOIP system would it be moved, and also it would mean an engineer to do the switch, and wire up you existing extensions.
I dont work for VirginMedia all opinions are my own. 39 years in IT.