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Reset all settings, then enter the special code (12345 by default). Mobile Master Codes for Nokia phones - Mobile Master nokia n139dl unlock extra quality
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To unlock the device for use with other carriers and improve its versatility, you can use these official and third-party methods: Carrier Unlock (Official) : Contact your service provider (e.g., Mobile Master Codes for Nokia phones - Mobile
Because you avoided sketchy flashing tools, your phone will still receive over-the-air (OTA) updates from Nokia. Go to Settings > System > System Update to ensure you have the latest security patch.