AT&T*, Orange, Telefonica, TeliaSonera, Verizon, Vodafone, Alcatel-Lucent,  Ericsson, Nokia Siemens Networks, Nokia, Samsung Electronics Co. Ltd., and Sony  Ericsson have defined the preferred way to ensure the smooth introduction and  delivery of voice and SMS services on Long Term Evolution (LTE) networks  worldwide.  
The above telecommunications industry leaders have jointly developed a  technical profile for LTE voice and SMS services, also known as the One Voice  initiative. The profile defines an optimal set of existing 3GPP-specified  functionalities that all industry stakeholders, including network vendors,  service providers and handset manufacturers, can use to offer compatible LTE  voice solutions.
 Open collaborative discussions have concluded that the IP Multimedia  Subsystem (IMS) based solution, as defined by 3GPP, is the most applicable  approach to meeting the consumers’ expectations for service quality, reliability  and availability when moving from existing circuit switched telephony services  to IP-based LTE services. This approach will also open the path to service  convergence, as IMS is able to simultaneously serve broadband wireline and LTE  wireless networks.
 By following the jointly defined technical profile, the industry can help  guarantee international roaming and interoperability for LTE voice and SMS  services, ensuring subscribers continuity of these vital services – all while  offering service providers a smooth and well-defined path to LTE.
 The objective of the initiative is to ensure the widest possible ecosystem  for LTE and to avoid fragmentation of technical solutions. LTE will, with this  initiative, not only serve as a broadband access for increasing data traffic,  but also for continuing voice and SMS services. Network operators will be able  to more quickly develop their customized LTE ecosystem in collaboration with  both network equipment vendors and device manufacturers. In addition, the  reassurance of global interoperability in an LTE voice landscape and the ability  to offer both broadband access and telephony services over LTE will create  strong foundations for future business.
 The profile for the initial solution has been finalized and is available  through the companies associated with this press release. The objective is to  hand over the profile and continuing work to existing industry forums.
 To view the technical profile, please visit http://news.vzw.com/OneVoiceProfile.pdf.
From Rethink Wireless:
One of the trickiest issues for early LTE deployers is uncertainty over how  voice and SMS services - still the key cash cows for most operators - can be  supported. Eventually, all these services will be carried over IP, using the IMS  (IP Multimedia Subsystem) standard, but only a few carriers, like Verizon  Wireless, are looking to deploy all-IP from day one. However, there is pressure  to accelerate the process and reduce the cost and risk of LTE/IMS for carriers,  and this is the objective of the new One Voice initiative.
  Some operators believe they will initially deploy LTE as a data-only network,  but most want to support voice and, even more importantly, SMS (which underpins  many cellco processes and customer communications). Faced with the risk that  large players might delay their plans until they have a strong route to voice,  One Voice has defined a profile based on existing 3GPP standards for IMS-enabled  voice.
    The work has initially emerged from Nokia Siemens, which was previously  trying to get wide industry support for its own interim voice over LTE solution,  VoLTE (which only worked with its own softswitches). The company's convergent  core marketing manager, Sandro Tavares, said One Voice should ease fears over  how voice will be deployed by resolving roaming and interworking issues at an  early stage. It is not creating a new standard, but aims to ensure compatibility  between networks and devices by creating a common profile, which defines an  optimal set of existing 3GPP functionalities for use by vendors and operators.  "There is no new standard," added Tavares. "It's just using what is there  already."
  NSN is already producing LTE equipment that complies with the new profile,  and so has a headstart in offering an important feature to early triallists -  which could boost its so-far low profile in LTE tests, dominated by Ericsson and  Huawei. However, the vendor will now hand its work to the 3GPP and GSM  Association so that other companies can work on and adopt the profile. So far,  it has signed support from most of the key operators that tend to wield  influence over cellular standards, apart from DoCoMo and China Mobile -  AT&T, Orange, Telefonica, TeliaSonera, Verizon Wireless and Vodafone are  there, plus a strong line-up of vendors. These are Alcatel-Lucent and Ericsson  on the infrastructure side and Nokia, Samsung and Sony Ericsson for devices. The  group needs to get the Chinese vendors on board to complete the set, as well as  Motorola.
The supporters of the initiative say this is their "preferred path" for voice  over LTE, though for carriers that do not want to move to IMS at an early stage,  there are other options available - namely open web-based voice; the stopgap  solution of Circuit Switch Fallback (also enshrined in 3GPP standards), where  the handset is forced off the LTE network onto 2G or 3G for voice calls; and  variations on the theme of using circuit switch over packet techniques. There  are two main approaches to this - MSC Voice, which is tied to a switch, with  NSN's VoLTE the most prominent example; and VoLGA, which is architecture  independent, and uses the UMA/GAN (Unlicensed Mobile Access/Generic Access  Network) protocol. This Kineto originated technology was originally adopted for  Wi-Fi/3G fixed-mobile convergence and as such did find its way into the 3GPP.  VoLGA does not require modifications in the LTE RAN or core, or the MSC, but  uses a separate gateway controller.
  Some of the One Voice supporters are already involved in VoLGA (though its  major carrier T-Mobile has not yet joined the new group). Steve Shaw, who heads  up corporate marketing for Kineto and VoLGA, believes that IMS is the way that,  ultimately, voice will be handled, but it has a long way to go before it is  usable, and so there will still be a role for several years for approaches like  VoLGA.
T-Mobile will I assume soon have to follow suit and fall in line otherwise they may have limited devices that are available and there will also be inter-operability issues.
Last week I attended a presentation by IET Berkshire on Voice Services over LTE, presented by Iain Sharp from Nortel. Even though this announcement came yesterday, Iain did say that IMS is the way forward for Voice over LTE. If interested you can see the presentation here.