When the forces of evil get together, it's usually bad news for the innocent bystanders, but this time it might be a good thing. Verizon and AT&T are getting together to take over the world make voice over LTE (VoLTE) work across carriers. That means you will soon be able to make calls from one carrier to the other using LTE rather than falling back to 3G technologies.
VoLTE is an all-IP calling technology that aims to improve voice quality and add additional services. For example, Verizon's Advanced Calling 1.0 program includes HD voice calls with VoLTE and video calling. The problem, however, it that VoLTE currently only works within a carrier network. So your connection defaults back to 3G calls if you connect to someone on another carrier, even if they are capable of VoLTE.
So why are AT&T and Verzon working together? Probably because making calls interoperable is good for both of them. It doesn't make sense to deploy a technology that only works on a single carrier. So they hear fewer complaints and you get better calls. AT&T-Verizon VoLTE interoperability is expected to be live sometime in 2015.