Top U.S. carriers are racing to roll-out Voice over LTE (VoLTE) calling, but there's one thing holding the new technology back. Since VoLTE uses the same LTE networks that transmit data, the service won't simply allow one customer on Verizon, for example, to hold a VoLTE call with a customer on another network. Instead, it's designed to easily work for customers who are on the same network. That's about to change; AT&T and Verizon on Monday announced a new partnership in which the two are working to allow VoLTE calls to work across networks.

Verizon said that engineers from both carriers are already performing lab tests and plan to extend that to field testing in the real world. Verizon said the goal is to eventually allow operators from around the world to work with its VoLTE network. That means one day someone in Germany with access to VoLTE will eventually be able to call the United States using the same high-quality IP connection. The partnership between Verizon and AT&T is just the start of that much larger globally connected VoLTE future that both carriers and the GSMA envision.

VoLTE promises to offer clearer phone calls by using an IP-based data connection instead of a traditional circuit-switched network connection. We've already seen AT&T, Verizon and T-Mobile roll out the service to same areas, while Sprint still seems to be holding back until it can offer a reliable experience. VoLTE is also limited to a handful of newer smartphones at the moment, likely because it's still not very widely available, though that will change as customers start to expect it on their new smartphones.