The first thing you need to do is cancel your subscription with us. See: How can I cancel/terminate my subscription?
Your SIM card will not become inactive immediately, you can still use the SIM card for the remaining time of your current package. Now you can initiate the porting process with your future mobile operator. As soon as we receive the request from your future mobile provider and all data is correct, we will release the porting.
The data in the customer portal must match the request from your future mobile provider. Once the number transfer to the future mobile provider is complete, you can destroy our SIM card.