Verizon Wireless does not plan to let voice calls placed over its LTE network roam on to its CDMA network when it introduces voice over LTE (VoLTE) smartphones next year.

"VoLTE will not hand down to 1X," Verizon Wireless product development executive Marjorie Hsu told Wireless Week, referring to Verizon's CDMA 1X Advanced network.

Marjorie HsuVerizon plans to hold off deploying VoLTE until its LTE network has enough coverage to ensure users won't experience high volumes of dropped calls, since VoLTE sessions won't be transferred to the CDMA network. "That's something we're managing very closely – introducing when we have enough coverage for those HD voice and video calls with VoLTE," Hsu says.

The company still expects to deploy VoLTE sometime in the first half of next year. Verizon has been aggressively expanding its LTE network and expects to match its 3G footprint with LTE service in 2013.  

It is technically feasible to transfer VoLTE calls to an operator's legacy network, but it requires operators to support Dual Radio or Single Radio Voice Call Continuity (SRVCC) technology. Supporting SRVCC adds an additional layer of complexity to both handsets and the underlying LTE network, a technology valued for its simplicity.

Verizon's LTE network currently handles only data traffic; calls placed on its LTE smartphones are routed over the company's CDMA network. That will change that when Verizon launches its VoLTE service next year. The change will not affect Verizon's current LTE smartphones, which use CS fallback to route calls over CDMA.

Verizon's VoLTE service will support more than just voice calls. Smartphones compatible with Verizon's VoLTE service will be able to handle a variety of rich communications services, such as video calls, multimedia messaging and instant-message style presence indicators.  The multimedia capabilities of Verizon's VoLTE smartphones will be a major selling point for the service, Hsu says.

The operator is currently working with handset manufacturers on VoLTE-compatible devices and is installing the necessary voice application services in its network to support VoLTE service.

Verizon isn't the only U.S. carrier working to deploy VoLTE. MetroPCS plans to bring its VoLTE service online next year and is currently setting up trials of the technology. MetroPCS is still evaluating handoff scenarios between LTE and CDMA with its equipment and handset vendors.