The government of Indonesia have recently issued a regulation which requires all mobile phones purchased overseas to be officially registered with the Indonesian Customs Office.
This regulation was established in order to restrict black market phones from entering Indonesia. Failure to do the registration process will result in the phone not being able to be used here in Indonesia – they will block di IMEI of the phone so regardless of whatever local sim card provider you use you will still not be able to use the phone unless the registration is complete.
In order to register your phone, you will need to complete the application form at https://www.beacukai.go.id/register-imei.html or download the app Mobile Bea Cukai via Play Store. Once completed, you will receive a QR code. Please note that you have to do this before you depart Indonesia. Upon your arrival at the airport in Jakarta you will need to proceed to the Customs Desk to show the customs officials the QR code so they can scan it and officially register your phone.