was not their core competence. These considerations gave rise to the concept of the hosted PBX. In wireline telephony, the original hosted PBX was the Centrex service provided by telcos since the 1960s; later competitive offerings evolved into the modern competitive local exchange carrier. In voice over IP, hosted solutions are easier to implement as the PBX may be located at and managed by any telephone service provider, connecting to the individual extensions via the Internet. The upstream provider no longer needs to run direct, local leased lines to the served premises. Manual PBX Many manufacturers provided manually operated private branch exchange systems in various sizes and features; examples are pictured here: System components A PBX often includes: Cabinets, closets, vaults and other housings. Console or switchboard allows the operator to control incoming calls. Interconnecting wires and cables. Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation. Microcontroller or microcomputer for arbitrary data processing, control and logic. Outside telco trunks that deliver signals to (and carry them from) the PBX. Stations or telephone sets, sometimes called lines. The PBX's internal switching network. Uninterruptible power supply (UPS) consisting of sensors, power switches and batteries. Current trends Since the advent of Internet telephony (Voice over IP) technologies, PBX development has tended toward the IP PBX, which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling, call forwarding, and programmable caller ID. As of 2015 ISDN is being phased out by most major telecommunication carriers throughout Europe in favour of all-IP networks, with some expecting complete migration by 2025. Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, PBXs have evolved into VoIP centers that are hosted by the operators or even manufacturers. Even though VoIP is considered by many people as the future of telephony, the circuit switched network remains the core of communications, and the existing PBX systems are competitive in services with modern IP systems. Five distinct scenarios exist: Hosted/virtual PBX (hosted and circuit-switched) or traditional Centrex IP Centrex or hosted/virtual IP (hosted and packet-switched) IP PBX (private and packet-switched) Mobile PBX solution (mobile phones replacing or used in combination with fixed phones) PBX (private and circuit-switched) For the option