Analog PBX
Analog PBXs use standard Plain Old Telephone Service (POTS) phones and copper wire. They are reliable, offer good voice quality and provide the basic features of a typical home phone (such as hold, mute, redial & speed dial) and can transfer calls between extensions.
Analog PBX keeps the extensions functioning even when the power goes out and the users remain connected. They are relatively cheap as they are simple and have limited options to expand or upgrade. However being less modular, analog PBXs are expensive to support, configure and upgrade. For instance, changing the location of an extension necessitates rewiring of punchboard by an experienced technician.
https://dealna.com/Article/Post/5646/Comparison-of-Analog-Digital-and-IP-PBX