Bluetooth device supports Bluetooth 2.1 so no keying PIN necessary.
All 3 Android phones Samsung Galaxy S3 with Android 4.1.2, HTC Sensation with Android 4.0.3 and Samsung Galaxy Nexus with Android 4.1.2 are running the same app - BluetoothChat app by Google targeting Android API level 15 (4.0.3). 3 apps on 3 phones were listening to incoming connections (Serial Port Profile I suppose) but only Samsung Galaxy S3 gets asked to allow or disallow pairing. I've tried doing the same test with each individual device and each pair of them but the result is the same.
Any ideas why this is happening?