service before they became available in hardware PBX equipment. Since introduction, updates and new offerings have moved feature sets in both directions. It is possible to get hosted PBX services that include feature sets from minimal functionality to advanced feature combinations. In addition to the features available from premises-based PBX systems, hosted-PBX: allows a single number to be presented for the entire company, despite its being geographically distributed. A company could even choose to have no premises, with workers connected from home using their domestic telephones but receiving the same features as any PBX user. allows multimodal access, where employees access the network via a variety of telecommunications systems, including POTS, ISDN, cellular phones, and VOIP. This allows one extension to ring in multiple locations (either concurrently or sequentially). allows scalability so that a larger system is not needed if new employees are hired, and so that resources are not wasted if the number of employees is reduced. eliminates the need for companies to manage or pay for on-site hardware maintenance. supports integration with custom toll plans (that allow intra company calls, even from private premises, to be dialed at a cheaper rate) and integrated billing and accounting (where calls made on a private line but on the company's behalf are billed centrally to the company). Hosted PBX providers The ongoing migration of most major telecommunication carriers to IP-based networks, coupled with the rise in Cloud Communications has resulted in a significant rise in the uptake of hosted PBX solutions. Mobile PBX A mobile PBX is a hosted PBX service that extends fixed-line PBX functionality to mobile devices such as cellular handsets, smartphones and PDA phones by provisioning them as extensions. Mobile PBX services also can include fixed-line phones. Mobile PBX systems are different from other hosted PBX systems that simply forward data or calls to mobile phones by allowing the mobile phone itself, through the use of buttons, keys and other input devices, to control PBX phone functions and to manage communications without having to call into the system first. A mobile PBX may exploit the functionality available in smartphones to run custom applications to implement the PBX specific functionality. In addition, a mobile PBX may create extension identifiers for each handset that allow to dial other cell phones in the PBX via their extension shortcut, instead of a PSTN number. IP-PBX An IP PBX handles voice signals over