I had several hangup calls from this number, but then I did get to answer one. They are an organisation who claim to seek a cheaper energy supply arrangement. Evidently they are associated with the group that supplies replacement LEDs for halogen downlights, and in agreeing to have your downlights replaced you also agreed to be contactable by their associates.

This number keeps on calling my silent unlisted number and when I answer they just hang up.  I called back but they didn't answer.