A TDM (Time Division Multiplexers) PBX is one of the most common types of voice infrastructures as it has been around the longest. A TDM PBX consists of proprietary, self-contained systems as it was designed before contemporary server technology was invented.
Involving a cabinet with numerous different boards that can perform certain functions, for example intercom functionality boards or analog extension boards, the TDM PBX is coming to the end of its life cycle. The TDM PBX boards are only compatible with systems from the same vendor as an overall architecture, locking in its users to use the same vendor for everything.
A TDM PBX requires dedicated staff to be able to manage it as well as extensive maintenance. It is mostly used by companies which have yet to update their network cabling.
The main difference between a TDM PBX and an IP PBX is that an IP PBX uses Internet Protocol to route calls whereas a TDM PBX uses physical switches. Additionally, an IP PBX is scalable, offers no vendor lock-in and can reduce telco costs drastically.