Is what was heard—a whispering voice calling out a name—and the sight of a brother searching for something, the spirit of the deceased father, or is it merely imagination?

There is no evidence from the Qur’an or Sunnah that the soul of the deceased visits their family members while awake and calls out to them; this is a form of delusion. However, seeing the deceased in a dream and conversing with them is a real occurrence and holds meanings for those who interpret dreams.