My AT&T Partner PBX (added to my collection in 2018)

• Partner expansion carrier which holds:
  1. One 306 Processor module (with 3 outside lines, 6 extensions)
  2. Three 306 (3 outside lines, 6 extensions per module) add-on modules
• Total system capability of 12 outside lines and 24 extensions (works with proprietary partner phones, or plain analog telephones – including rotary dial sets).

This is not in use for anything currently, though I keep it around in case I needed a small PBX; for example it would work well for a mobile phone/modem demonstration at a hamfest/collector show, it’s fairly small and low power (in the base processor/stand-alone 306 config anyways).