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

There are no earth-bound souls. When the souls that are not good die they go entirely away from this earth and so cannot influence anyone. They are spiritually dead. Their thoughts can have influence only when they are alive on the earth… But the good souls are given eternal life and sometimes God permits their thoughts to reach the earth to help the people.

‘Abdu’l-Bahá, Daily Lessons, Received at ‘Akká, 1979 ed., pp. 35-36

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.

Shoghi Effendi, Lights of Guidance, p. 514