With the Global mobile Suppliers Association (GSA) reporting 100 mobile carriers in 71 countries now supporting HD voice, AT&T and Verizon promising VoLTE this year, and Sprint claiming it will deliver HD voice nationwide by July, it is time to look forward to the next phase of implementation: Seamless HD voice calling across networks. It's a headache involving SBCs, IP exchange (IPX), and transcoding.

To date, all carriers that have launched HD voice service include the caveat that both callers must be on the same wireless network, as well as using an HD voice capable phone. For example, if you were a France Telecom customer in France making a call to a T-Mobile customer in Germany, you wouldn't get an HD voice between the two parties at the present time. Both carriers have to agree to either peer -- exchange HD voice calling traffic as equals -- or use a third party IPX (IP eXchange) service to make an HD voice call from end-to-end.

Carriers have been conducting a lot of low profile work to establish IPX service points in the mobile world, with BT and iBasis the ones that first come to mind. IPX enables mobile carriers to enable such things as HD voice calls across networks, VoLTE calls across networks, and LTE roaming services. Everything revolves around the use of IP to move voice and data between carriers, with the IPX as the centralized "meet me" style point where multiple carriers can move bits around without having to establish individual peering agreements on a one-to-one basis.

In the broadband world, IntelePeer and others have been promoting G.722 and SIP call exchanges. Statistics for the success of such exchanges is scarce, with no data on the number of reachable end-points able to receive end-to-end SIP calls or the total number of participating carriers.

If interconnection between like types of calls, such as a mobile HD voice call using the AMR-WB codec, wasn't enough work for one day, there's the added complication of transcoding (translating) between different types of codecs. Nearly all of the mobile world users the AMR-WB codec at this time, including 2G GSM, 3G and VoLTE calls. The U.S. exception to the rule is Sprint, which uses the 1X Advanced on its CDMA network to deliver HD voice. When Sprint rolls out VoLTE some year in the future or if it simply wants to exchange HD voice calls with AT&T, T-Mobile US, Verizon and the rest of the world, someone will have to transcode between its format and AMR-WB.

Transcoding between G.722 and AMR-WB is also something that has been discussed a bit by carriers, including BT and France Telecom. Both Deutsche Telekom and France Telecom have the need to convert HD voice calls from their respective broadband networks to their mobile networks. In the enterprise space, AT&T and Verizon will have similar needs as businesses require better voice quality for conference calls and Smart Voice applications.

Sooner or later, carriers will have the ability to establish seamless end-to-end HD voice calls, regardless of network or codec type. How much later is the question, given the relatively sedate pace of HD voice service turn ups around the globe.