What is a significant difference between a TDM PBX and a VoIP PBX$?$

A TDM PBX is supplied as vendor$-$specific hardware, while a VoIP PBX can be implemented as software.

A VoIP PBX requires a separate data channel for each call, while a TDM PBX does not.

A TDM PBX can only support voice mail, while a VoIP PBX cannot.

A TDM PBX uses the Internet for all calls, while a VoIP PBX uses the PSTN$.$