IMS was designed in a very a general way. As a service network which is access independent, supports multiple identities and it is very expensive to implement. Sometimes less is more – at least in the beginning. That’s also why we have the VoLTE – a minimum IMS profile which reduces the scope into something manageable and more practical.
As the VoLTE has been around for a couple of years already and works fine, the early adopters are looking for more – HD Voice, EVS, WebRTC and other technologies. Recently T-Mobile USA (DIGITS) and AT&T (NumberSync) announced their solutions for multi-device feature. Not all the features are always successful or truly practical – but this time I’d think more operators will follow. Btw. with IoT to come I wonder when exactly we will see a first virtual entity (agent) seamlessly moving from one devices to another 🙂