Quietly, gradually, but clearly Apple is building platform lock-in into its iOS products, with some features that are deeper than just a rich third-party software ecosystem. FaceTime Audio is the latest of these, VoIP calling built on the back of its FaceTime video chat service, which is tightly integrated to the phone app to make placing free international calls almost a pleasant surprise for those looking to connect with far-flung loved ones. FaceTime Audio joins iMessage as another reason to sign on with the Apple camp and get an iPhone or iPad device, and then never to depart again. It’s a little odd to see it arrive so much later than FaceTime’s video calling feature, but the reversal of feature rollout makes a lot of sense ; audio-only calls are uncomfortably close to standard phone calls, which is still one of the sole remaining areas that carriers control.