Using Voice over Internet Protocol (VoIP) calling applications on your mobile phone can save you money if you make a lot of international calls or travel out of the country often. iPhone users have several options when deciding what app to download.
Citel PBXgateways are installed at the central PBX location, and EXTenders are deployed at branch offices, call centers, or at home offices over managed IP connections, DSL, cable, or analog circuits