A new update has started rolling out to T-Mobile Lumia 530. This update is specific to the T-Mobile version and adds Wi-Fi calling support in Airplane mode. It is still Lumia Cyan firmware and Windows Phone 8.1 as indicated by the OS version no. below.
Latest software release for Nokia Lumia 530 is Lumia Cyan (01068.00023.14391.54104 / 8.10.12400.899)
• Wi-Fi calling is now supported also in airplane mode.