Outside of the Bounty of the Holy Spirit all that thou hearest concerning mesmerism or trumpet communications from the dead are sheer imagination.

‘Abdu’l-Bahá, Tablet to Ella Goodall Cooper: Daily Lessons Received at Akka, p. 85, 1976 ed.; in Lights of Guidance, no. 1761

Regarding the materialization of spirits through mediums: A person finding himself in a state of trance, or unconsciousness, is like one who sleeps; whatever he feels and sees he imagines to be matter and of material things, but in reality they are wholly immaterial.

‘Abdu’l-Bahá, Daily lessons Received at 'Akká p. 82

Briefly, there is no question that visions occasionally do come to individuals, which are true and have significance. On the other hand, this comes to an individual through the grace of God, and not through the exercise of any of the human faculties. It is not a thing which a person should try to develop. When a person endeavors to develop faculties so that they might enjoy visions, dreams etc., actually what they are doing is weakening certain of their spiritual capacities; and thus under such circumstances, dreams and visions have no reality, and ultimately lead to the destruction of the character of the person.

Shoghi Effendi, Lights of Guidance, p. 515

He would advise you to consider the voice you heard a phenomenon which might be your subconscious mind, might be some psychic influence, but whatever it was you should not let it disturb you and certainly not place much importance on it. No one knows what the future holds for him, or to what degree he is spoiling it or creating it; therefore the thing to do is one's daily best and let the future take care of itself. It would be very unwise for you to let this experience of a voice-the origin and purpose of which you have no way of knowing-influence you in any way or to set any store on its observations.

Shoghi Effendi, Lights of Guidance, p. 515

Regarding your question: In His chapter on 'Visions and Communications with Spirits' in 'Some Answered Questions' the Master evidently desires to point out that there can be, under certain rare circumstances, such as those experienced by the Prophets, communion with some soul gone before into the invisible world, but that most of this type of experience which people often claim to have with departed souls is nothing but the product of their own imaginations-however real it may seem to them to be . . . Truly mystical experiences based on reality are very rare, and we can readily see how dangerous it is for people to go groping about in the darkness of their imagination after the true thing. That is why, as you point out, we are warned against all psychical practices by the Master. If we are going to have some deeply spiritual experience we can rest assured God will vouchsafe it to us without having to look for it.

Shoghi Effendi, Lights of Guidance, p. 514

The Guardian thinks that it is best to assume that generally speaking when people claim they are receiving messages or communications from the Master or Bahá’u’lláh, etc., it is a psychic experience or their imagination, and that they are not in real contact with them. These Holy Beings have the channels of the Cause through which to guide us. They do not need to go outside these and send individual revelations.

Shoghi Effendi, Lights of Guidance, p. 515