to call from IP network to the circuit-switched PSTN (SS7/ISUP), the hosted solutions include interconnecting media gateways. Home and small-business usage Historically, the expense of full-fledged PBX systems has put them out of reach of small businesses and individuals. However, since the 1990s many small, consumer-grade and consumer-size PBXs have become available. These systems are not comparable in size, robustness or flexibility to commercial-grade PBXs, but still provide many features. The first consumer PBX systems used analog (POTS) telephone lines, typically supporting four private analog and one public analog line. They were the size of a small cigar box. In Europe these systems for analog phones were followed by consumer-grade PBXs for ISDN. Using small PBXs for ISDN is a logical step, since the ISDN basic rate interface provides two logical phone lines (via two ISDN B channels) which can be used in parallel. With the adoption of VoIP by consumers, consumer VoIP PBXs have appeared, with PBX functions becoming simple additional software features of consumer-grade routers and switches. Additionally, many telecommunications providers now offer hosted PBX systems where the provider actually hosts the PBX and the phone handsets are connected to it through an internet connection. Open source projects have provided PBX-style features since the 1990s. These projects provide flexibility, features, and programmability. Launched in 1997, IP-PBX took business communication to the next level. IP-PBX is a PBX phone system that uses Internet Protocol (IP) data networks to manage the routing and switching of calls as well as to handle messaging. Virtual hosting resulted in significant cost efficiencies. Apart from offering advanced features (like voicemail), the system could also use a VoIP gateway to connect to traditional PSTN lines. This gave the user the option of continuing with the same carrier. Over time, improvements in the user interface and call quality, along with cost benefits, added to the popularity of IP-PBX among small businesses. PBX functions Functionally, the PBX performs four main call processing duties: Establishing connections (circuits) between the telephone sets of two users (e.g. mapping a dialed number to a physical phone, ensuring the phone isn't already busy) Maintaining such connections as long as the users require them (i.e. channelling voice signals between the users) Disconnecting those connections as per the user's requirement Providing information for accounting purposes (e.g. metering calls) In addition to these basic functions, PBXs offer many other calling features and capabilities, with different