Talking with the dead has been a time honored endeavor, and all human cultures have terms for and tales about this common human experience. Most of the tales are told by those who recently lost a loved one, and the bond continues for a time due to unfinished business and concerns that the recently deceased has about those still living. The most famous tales are told about hauntings, where a particular spirit hangs about a location such as a house, wanting some kind of justice done or lingering due to past attachments. Seances, where the dead are deliberately called, are a form of The Call, and as such fall under the rules whereby more than the loved one called can arrive. Thus, a seance can bring forth the spirit called, other restless formerly human spirits, and various visiting aliens who may or may not materialize depending on the setting.
A group of people fervently asking a spirit to manifest, to communicate with them, will reach that spirit by one means or another. That the spirit does not manifest is due not to The Call failing to reach this spirit, but to reluctance or disinterest. If the spirit has incarnated again and is caught up in a new life, they may dismiss The Call to them without giving it a thought. If the spirit was tired of the talk about a subject or obligations that were imposed by those who are now calling it, the spirit may turn its back and refuse to participate, free at last in death. If the spirit likewise has been longing to communicate, it may take the opportunity, or may decide that a one-on-one with this or that individual is preferable and make their own arrangements later, haunting the one they wish to communicate with.
When The Call is given to a spirit that incarnated as a human, this spirit can being called for its most recent life, the most recent incarnation, or can be called for one of its past lives. All are remembered by the spirit as vividly as the most recent incarnation. However, this setting is virtually begging for interference by Service-to-Self aliens looking to influence humans at every opportunity. Most calls to spirits who incarnated famous or influential humans in the past are done not for altruistic reasons, but for personal gain. Where did you bury the gold, what was the missing piece to the puzzle that would allow an idea to be patented, where are the manuscripts stored, etc. Thus The Call has been given to the Service-to-Self, who are notable for lying and leading astray. Pretending to be the spirit called is easy, and feeding the self-serving tendencies of the humans giving The Call easier still.
Identity cards cannot be checked at seances, so anyone and anything might manifest, pretending to be the spirit called!