You mean her future phone even can't have a net connection and she would merely be using the phone as an interface to communicate with Asuna? Just like a phone is used also by people when you make a phone call to somebody. You don't actually upload your consciousness to the recipient's phone, you merely send your voice over there and receive the other person's voice in return. Yui's involvement with Asuna phone would be a bit deeper but nothing fancy, really. I'm sure even today you can download some shady app from the Internet onto your phone that lets some foreign bastard do whatever he wants with your phone, as long as the net connection is on. Monitor the surroundings with the cameras and microphone, use the IR to change your TV channels if he recognizes your TV brand, whatever.