As long as both of the LAN PBX controlling servers have an Internet connection, they can be set up to merge their extensions. So a person in a San Francisco office could dial an extension to talk with a colleague in London, and then conference that call with a person in the same building, and again with another colleague in New York. The total cost of this call would only be the cost of the data packets being transmitted over the Internet, which means next to nothing. Note that, to the end user, there is no discernible difference. Fourth, because a LAN PBX is designed to run over an existing data network, the phones don't even need to be separate pieces of hardware.
Soft phones use a computer's network connection, sound card, and a microphone-speaker setup.
Using soft phones is cheaper than purchasing the telephone hardware for either traditional or LAN PBX systems. This solution also frees up valuable desk area in cramped quarters. When physical IP phones are used, the cost per phone is usually hundreds less than those used on traditional digital PBX systems. Fifth is the consolidation of IS services within large corporations. Banks and other companies with many branch offices need large support staffs for both the telephone and data networks.