With Voice over IP you are not tied to any specific location, because all calls are routed via the Internet, and not through static copper wires. This means that you can have a single phone number or a range of numbers that you can point at VoIP extensions, and these extensions can be anywhere in the world that you have a VoIP phone/device.
For example you could have a London phone number and send calls to VoIP phones in different offices, homes, or smartphones, and have all those phones ringing at the same time. You can even set the times of day that individual, or groups of phones will ring. If someone moves from their existing location, they simply unplug their device and reconnect it to the Internet in their new Location.
You can even choose which of your numbers each extension will use as an outbound Caller ID. This flexibility allows you to utilise your staff resources effectively and efficiently. You can also configure the same extension number to multiple devices so that if you get a call it can ring you on your VoIP phone in your office, home or on a smartphone app.