>By comparison, only around 12 per cent of the carrier's monthly subscription customers (as opposed to prepaid customers) still use 2G handsets, making the spectrum AT&T has set aside for that service a growing liability.<
This is a bit of untruth. What you mean to say is that 12% of subscribers use phones that are limited to 2G. But even 3G and 4G phones are 2G capable.
Even 3G or 4G phones still make voice calls over 2G. Phones support multiple bands and multiple technologies. A 4G phone falls back to 3G in areas that have no 4G coverage, and to 2G when there is no 3G.
However, they ALL USE 2G for making voice calls. The carriers have billions invested in the 2G switches and infrastructure, and will HAVE TO USE THAT until 4G and VoLTE becomes ubiquitous. They are not going to remove any 2G equipment for some years, and even then only reduce capacity, not eliminate it. And with AT&T so far behind VzW in the LTE deployment, they will be supporting 2G for some time. Beyond 2017, I should think.
And another correction, T-mobile in the U.S. has committed to deploying LTE, although they haven't started yet.