AT&T and Verizon to connect cross-carrier VoLTE calls by 2015

Soon customers on AT&T and Verizon will be able to call each other with HD voice quality thanks to a partnership that will bring cross-carrier compatibility to each network's Voice over LTE (VoLTE) service. Currently, the higher quality HD voice benefits of VoLTE calls are limited to customers who are in a VoLTE network and who own a VoLTE compatible device where the calls are made to another customer on the same network.

source: androidcentral