Does PBX need internet?

As someone interested in understanding the technical requirements of PBX systems, I have a question. Does PBX require an internet connection to function properly? I've heard some people mention that it can work without it, but I'm not entirely sure. Can you clarify if an internet connection is necessary for PBX to operate, and if so, why? I'm looking to set up a PBX system for my business and want to ensure I have all the necessary components in place.