Check the type of your agreement with your current operator. The agreement may be valid until further notice or for a limited time period and it may concern subscriptions paid in arrears (post paid) or in advance (prepaid). You can port a number of a prepaid service only if you are registered and identified as the subscriber of the service. The number of an anonymous prepaid service can not be ported.
You can see the type of your agreement and the amount of the monthly fees in your agreement and in the operator's price list. Most operators have also indicated the expiry date of a fixed-term agreement on the telephone bill. Contact your operator's customer service, if necessary.