The place that I’m working uses VoIP telephones. What that means is that the call is routed, at least here in the office, over the internal computer network, not over a dedicated set of telephone cabling. If the computer network fails, so do the telephones.
There are some fringe benefits: the system is much easier to upgrade and replace than a dedicated system would be, computers that need a wired connection can plug directly into the telephones, saving one from scrabbling under the desk, and because the telephone is connected to the network using an ethernet interface, and the extension number is assigned by MAC address, one could unplug a telephone and move to a different office and the extension number would move with you. For the network administrators, life is much easier. A central directory of telephone numbers is stored on a server and downloaded to each telephone.
For the users, however, the system is frustrating. Call quality is poor - much poorer than a telephone would be, and this is especially noticeable when making long-distance calls or when the network is busy. It is also impossible to enter one’s own speed dials without access to the web interface, which is kept a closely guarded secret by the IT people. Knowledge is, after all, power.