A live person is forbidden from using necromancy to communicate with a dead person.
But is the reverse true?
Is the dead person permitted to initiate the contact?
If the live person contacted the dead person through forbidden necromancy, is the dead person permitted to respond?
Samuel's response to Saul is not a proof example because his warning that Israel would be defeated was pekuach nefesh.
The Talmud says that the dead aren't bound by mitzvos
גמרא נדה ס"א::ה' וְרַבִּי יוֹחָנָן לְטַעְמֵיהּ דְּאָמַר רַבִּי יוֹחָנָן מַאי דִכְתִיב {תהלים פ"ח:ו'} בַּמֵּתִים חׇפְשִׁי כֵּיוָן שֶׁמֵּת אָדָם נַעֲשָׂה חׇפְשִׁי מִן הַמִּצְוֹת.
And Rabbi Yoḥanan conforms to his standard line of reasoning in this regard, as Rabbi Yoḥanan said: What is the meaning of that which is written: “Set apart [ḥofshi] among the dead, like the slain that lie in the grave, whom You remember no more” (Psalms 88:6)? Once a person dies, he becomes free [ḥofshi] from the mitzvot.
Rambam codifies it lihalacha
רמב"ם כלאים י':כ"ה מותר לעשות מן הכלאים תכריכין למת, שאין על המתים מצוה It is permitted to make shrouds for the deceased from kilayim, for the deceased are not obligated in any mitzvot.
So it would seem that one is certainly permitted to respond to a posthumous summons.
I'm unsure about initiating as perhaps the dead are only off the hook regarding personal mitzvos but not regarding tripping up others ("lifnei iver").
In any event the above is all presumably unnecessary to allow for either responding or initiating as the prohibition is for a living person to summon a spirit—not for a spirit to initiate or respond.
An example where it's done: the work "She'elot u-Teshuvot min Ha-Shamayim", Responsa from Heaven, is a 13th-century collection of almost seventy halachic questions, posed by Rabbeinu Yaakov of Marveges and answered by members of the heavenly Yeshiva. One of the members of the Yeshiva is implied to be the deceased R. Yitzhak al-Fasi (Siman 8).
