A computer, including an ethernet interface, a suitable LInux system, Asterisk.
You may also need circuit switechd digital telphony cards, or analogue ones, the Dahdi software, and just possibly other hardware and digital telephony related software, but there is far too little information to be certain of that, and of what kind you would need.
What you probably most need is to go to asteriskdocs.org/ and read the book that is there.
What kind of existing PABX? How will the PSTN connections be presented to Asterisk? What inter-PBX trunk options are available on the existing PABXes? What traffic volumes and number of lines are involved? What features, beyond basic call setup are required?