Warren, on the other hand, isn't a big proponent of CS Fallback.
"I am hoping [the first launches] will give a strong indication that within the next 12 to 18 months operators can take the step directly into VoLTE and not encumber themselves with a CS Fallback launch," said Warren.
Proponents say VoLTE will result in better voice quality thanks to lower latency and HD Voice becoming a defacto standard. There is nothing in the VoLTE specification that makes implementing HD Voice mandatory, but most operators are still expected to do that, according to Warren.
The improved quality HD Voice offers is possible thanks to AMR-WB (Adaptive Multi-Rate - Wideband), a speech-compression algorithm that doubles the range of voice frequencies transmitted.
To take full advantage of the improvement HD Voice can offer, users also need phones and headsets with good sound quality.
"More and more people are using headsets, and you can really tell that there is a difference in quality. There are lots that are really bad," said Ljunggren.
However, HD Voice isn't a VoLTE exclusive, and has already been implemented in over 40 3G networks, according to GSA. Lower connection times should give VoLTE an edge over CS Fallback, but the technology also introduces new potential problems, according to Ljunggren.
That's because the implementation of VoLTE also means telephony traffic is moved from a circuit-switched world to one where everything is IP-based.
"Previous mobile networks have been optimized for telephony and SMS, while LTE has been optimized for offering broadband connections that are as fast as possible," said Ericsson.
The challenge is to ensure the quality of the voice service on that data-centric network, including at the edge of a cell or network where the available bandwidth is limited, according to Ericsson. The key to doing that is prioritizing voice over data, he said.
"From the LTE device, telephony is sent using one radio bearer or pipe and data is sent using another pipe, and if a conflict arises, the telephony gets priority," said Ericsson.
Getting the quality right will be important to operators. The disaster scenario is that the quality and coverage is not as good as the old telephony systems, according to DePuy, who nonetheless is still optimistic about the outlook for the pioneering operators.
"I actually do think they will be successful. Technically it will work," said DePuy.
The use of traffic prioritization opens up discussions on network neutrality, but Ljunggren isn't too worried.
"As long as we offer other services the same possibilities to use QoS and priority, I don't see a problem with using it for VoLTE," he said.