Grasshopper works on top of your current phone system, so you’ll still need to have an account with a provider to get calls (cell or landline). Only calls that are handled by Grasshopper are billed to your Grasshopper account. Your local and long distance carriers will continue to bill you for any service that they provide to you. If you transfer your local number(s) to Grasshopper your local carrier will be Grasshopper.
Grasshopper forwards calls from your new number to an existing physical phone with its own number, for which you need a third-party carrier to host that number.