Clearwire already sells VoIP for use on WiMax home modems, and it may offer mobile voice through Sprint Nextel, using the cellular carrier as a mobile virtual network operator, said President and Chief Architect Barry West. Like Verizon, Clearwire plans to add value to its VoIP offering by bundling it with other services. But the company can also make money from third-party providers, West added. The company will sell access to the higher layers of its software stack, on a non-discriminatory basis, so VoIP companies can get the low latency they need for high-quality calls, he said.

Sprint Nextel officials weren't immediately available for comment.

Yet for the mainstream carriers, turning voice into just another application on a packet-based network may still be years away.

To begin with, their 3G networks will stay in place as LTE is gradually deployed. Even Verizon, expected to be the first major operator to commercially offer LTE, said its EV-DO (Evolution-Data Optimized) infrastructure will still be around in five to seven years. Until LTE is available everywhere, handsets are likely to have two radios so they can fall back on 3G for voice.

In addition, several major network vendors have teamed up to develop a specification for handling VoIP calls like traditional voice traffic. The VOLGA (Voice Over LTE via Generic Access) standard will let carriers continue to use the MSCs (mobile switching centers) already in their networks to handle voice calls, said Steve Shaw, vice president of corporate marketing for Kineto Wireless. Kineto is a member of the VOLGA Forum alongside Alcatel-Lucent, Ericsson, Huawei, and other vendors.