VoIP functions on broadband networks using packet switched telephone networks (PSTNs), unlike traditional calls on circuit switched networks. PSTNs are more efficient than legacy systems (circuited switched networks), and information can be routed in packets over optimized paths. VoIP allows IP calls over mobile networks (3G/LTE) or fixed networks, such as wireless hotspots, Wi-Fi, and worldwide interoperability for microwave access (WiMAX) networks. VoIP bypasses the originating mobile carrier's telephony network and allows inexpensive calls on mobiles through the ISP networks. It also offers low-cost international calls, and roaming calls, without the need for extensive infrastructures such as PCs and DSL modems.