Most commercial VOIP providers will let you keep your old number when setting up VOIP. But the process isn't all that smooth. You're going to have to sign a form allowing the provider to take your old number. It can take 30 days or more for that old number to work on the new VOIP system, so the provider will give you a temporary number to use until then. It's much easier to set up VOIP with a new number. So consider this before assuming you can easily move your number around from provider to provider.