It is a device capturing and controling all the signals, which are being created by the whole system, especially the made and received phone call signals. The device is mostly used for connecting regular phone system with the Internet and backwards.
Virtual PBX or Hosted PBX is offered by a telephone company and can free your business from investing in the costly hardware or a complex business phone system. Virtual PBX means that every service is processed via a telephone company offering a service with their own hardware and software.