1. If you're paying for 12 months line rental as a lump sum payment, and then leave for another provider within that 12 months, then they will keep your money.  You will lose any remaining months.  You will have to pay the new provider for the service.
 2. Again, this will be in the contract.  Usually, you can take the service with you.  The service just carries on for the remainder of the contracted period.  Check what happens if they are unable to provide the service at the new address.  The terms may also be different if the provider has to install a new line at the new property.