If you don't currently pay for Sideline (no active subscription) your account will be placed in what we call a limited state. Accounts in a limited state still have the option to port their number.
If you are porting out a Sideline number that you chose during registration, you will need to pay the one-time port out fee ($19.99) in order to unlock the number and receive the Account number and PIN.
If you originally ported the number into Sideline from another service, you are not required to pay the port-out fee since the number belonged to you when you registered.
To begin the port-out process: Sideline Settings > Support > Port-Out Number.
If you have any further questions, please submit a request.