When you sign up to a MyNetFone plan offering a free month of service, you will receive the free month credited to your account with your first invoice. The credit will generally be identified as 'Service Charge Credit' or 'Goodwill Bonus'.
This means that once you have signed up, you will be billed for the first month of service. The initial payment will be withdrawn from your preferred debit/credit card. With the first email invoice you receive, you will notice the value of 1 month of service credited to your MyNetFone account. In effect, this will make your second month of service free as the surplus credit means no money will have to be withdrawn from your account that month.