This year the number of mobile networks that are compatible with HD Voice has grown to 36, almost triple what it was before. That is still a relatively low number, compared to, for example the 152 HSPA+ (High-Speed Packet Access) networks that are in commercial use, according to data from industry organisation GSA (Global mobile Suppliers Association).

But the interest for HD Voice is picking up, GSA president Alan Hadden said.

"In the early days, HD Voice support was mostly limited to mobile networks operated by Orange, but now other carrier groups are also rolling out the technology," said Hadden.

That is creating a positive trend.

"The launches also create more interest from other operators that we are talking to continuously," said Jan Derksen, head of technical marketing at Ericsson Networks.

Revenue from data services is growing, but mobile operators are still dependent on voice calls.

"There is a business case for HD Voice. Surveys show that people talk longer and also make more calls," said Hadden.

Apple and RIM yet to embrace HD voice

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, resulting in sound quality that French operator and HD Voice pioneer Orange in the past has compared to FM radio.

Operators also have to implement a technology called transcoder-free operation, which is needed for HD Voice phone calls to travel across the mobile network with unchanged quality, according to Derksen.

But an empty network isn't very useful. The growing interest among operators is helped by the availability of more phones that can make and receive HD voice calls.

A year ago users could only choose among a handful of phones, but now they have at least 60 models to look at, including Samsung's Galaxy S II; Nokia's new Windows Phone-based Lumia devices; and Sony Ericsson's Ray. Still missing from GSA's list of compatible devices are Apple's iPhone and RIM's BlackBerry.

Besides phone availability, the usefulness of HD Voice is limited by the fact that calls can only be made between users on the same network in the same country. The way networks talk to each other has to change for that to be possible, and Ericsson is aiming to deliver the underlying technology during the second half of next year, according to Derksen.

VoLTE and HD Voice

"That will be a big boost," said Hadden.

Next year, the first operators will probably to go live with VoLTE (Voice over Long-Term Evolution), and they are expected use HD Voice, as well, Derksen said.

To help raise awareness about the technology, the GSM Association earlier this year introduced an HD Voice logo that can be used by both operators and phone makers.

To be able to use the logo, phone makers need to do more than just implement the AMR-WB codec. If the microphone and speaker acoustics and the on-board signal processing are not up to the same standard then the HD voice quality will not be the best possible.

To be compliant, the GSMA also requires improved speaker and microphone acoustics and also better audio signal processing on the device to remove noise, a spokeswoman said via email.