It is not common to see Verizon and AT&T work together, they are competitors after all, but in this case it all makes sense. Both carriers have joined forces to make VoLTE interoperable. Basically what this means is that voice calls made over the LTE network will not be confined to either AT&T or Verizon when it all gets put into place, rather subscribers will be able to call their counterparts on either network without having to come down from the LTE network.
VoLTE takes voice calls and puts it on the LTE network. This in turn provides subscribers will improved call quality as well as some other features. For example, Verizon Advanced Calling 1.0 brings voice calls in HD as well as video calling straight within the phone app.
There’s a caveat though. Advanced Calling 1.0 can only really be used when calling someone who also happens to be on Verizon. The features won’t be available when calling someone on another network, like AT&T.
But the partnership between these carriers will change this. They’re the only major carriers that have joined forces for this as of now but its possible that T-Mobile and Sprint might jump on board as well in the future.
Verizon and AT&T expect VoLTE interoperability across their networks to go live by next year.