Operator alternatives
There has been a standard in place for over ten years, which is supported by the infrastructure that all the operators already use that could put the over-the-top players out of business tomorrow. Called IMS, it uses an extremly simple concept. I dont want to communicate with your land-line, your mobile or your PC, I want to communicate with you. So if you have indicated which device you can be reached on, then when I try to contact you (using your personal identifier), the infrastructure will negotiate the path that best supports the form of comms I have requested (voice, VOIP, video etc.) to the most available and capable device connected to you.
The classic sales example is the guy who starts out working in his garden. He doesn't have his mobile with him but his wrist watch can recieve SMS texts. As he walks back into his house, his watch informs the houses cordless phone system that he is in range and can now recieve voice calls. As he goes into the TV room, his watch contacts the telly, and now he can recieve a fully interactive video call.
There are various theories about why this hasn't been widely adopted yet, my personal favourite is that the business people at the operators have only barely managed to get their heads around 2G and clearly have not grasped the impacts of data on their business yet. One of them will, hopefully soon, and once one takes the risk and tries it, the rest will jump on the bandwagon.