equipment from several manufacturers is on site, the use of a standard protocol is required. QSIG – for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits. Interfaces for connecting PBXs to trunk lines include: Internet Protocol – H.323, SIP, MGCP, and Inter-Asterisk eXchange protocols operate over IP and are supported by some network providers. ISDN – the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2-circuit capacity) or Primary (24- or 30-circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections. RBS (robbed bit signaling) – delivers 24 digital circuits over a four-wire (T1) interface standard POTS (plain old telephone service) lines – the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call (commonly called glare). Interfaces for collecting data from the PBX: File – the PBX generates a file containing the call records from the PBX. Network port (listen mode) – an external application connects to the TCP or UDP port. The PBX streams information to the application. Network port (server mode) – the PBX connects to another application or buffer. Serial interface – historically used to print every call record to a serial printer. In modern systems a software application connects via serial cable to this port. A data record from a PBX or other telecommunication system that provides the statistics for a telephone call is usually termed a call detail record (CDR) or a Station Messaging Detail Record (SMDR). Hosted PBX systems Virtual PBX systems or hosted PBX systems deliver PBX functionality as a service, available over the public switched telephone network (PSTN) or the Internet. Hosted PBXs are typically provided by a telephone company or service provider, using equipment located in the premises of a telephone exchange or the provider's data center. This means the customer does not need to buy or install PBX equipment. Generally the service is provided by a lease agreement and the provider can, in some configurations, use the same switching equipment to service multiple hosted PBX customers. The first hosted PBX services were feature-rich compared to most premises-based systems of the time. Some PBX functions, such as follow-me calling, appeared in a hosted