Long-Term Evolution (LTE) is the 4G evolution of the 3G Universal Mobile Telecommunications System (UMTS). Notably, it replaced both UMTS, which descended from the 2G Global System for Mobile Communications (GSM), as well as the 3G Code-Division Multiple Access 2000 (CDMA2000), which was the main competitor to UMTS. For the first time, this meant that cell phone networks worldwide coalesced around a single protocol, removing the need for world travelers to carry a phone with dual radios. One reason LTE was able to achieve this was by supporting a mixed-mode network, where CDMA2000 (or UMTS) was used for voice calls and LTE for data communications. This provided a seamless upgrade path for cell phone operators. Over time, most carriers eventually switched to Voice over LTE (VoLTE).