actually, simultaneous voice and data on Verizon is just rolling out now... it's because of how Verizon implemented their system. It's not necessarily CDMA vs. LTE vs. GSM.
AT&T used one antenna for voice and one antenna for data. That's how you get simultaneous. But Verizon wanted to ensure better and more consistent calling so both antennas are dedicated to voice during a call.
But it's rolling out later this year...
VoLTE - Verizon Wireless
The next evolution in wireless calling will be available for Verizon Wireless customers in the next few months. VoLTE – or Voice over LTE – is slated to roll out on Verizon’s nationwide network later this year.
Verizon has always been focused on a standards-based, nationwide implementation, supported by a tangible volume of handsets. The company will have a robust line-up of VoLTE-enabled smartphones available when the service launches. And, more and more customers across the country will be able to use VoLTE as select handsets will be VoLTE-enabled through a software update leading up to the roll out of the service, with new devices VoLTE-enabled at the time of purchase later this year.
For a customer to make or receive a VoLTE call, both parties must have VoLTE-enabled devices and be in an area where VoLTE is available.
Verizon’s VoLTE will also offer video calling options, including making and receiving video calls directly from contact lists. As part of the VoLTE video calling experience, customers have the ability to change their calls instantly from voice-only to voice and video. The rollout also sets the stage for future enhancements through Rich Communications Services (RCS), which enable things like including large file transfer, more robust group messaging, and more location sharing.