We do not find a prohibition as such against merely speaking to the deceased. The text you mentioned about King Shaul involved the prohibition of "Ov", as 1 Shmuel 28:7 relates that they went to a woman who was a "Ba'alas Ov". The Torah states (Devarim 18:11) that one is not allowed to ask from Ov. Rashi there explains that this is a sort of witchcraft where the witch speaks from her armpit and raises up the dead. The Torah also mentions in the same verse the prohibition of "seeking the deceased". The Gemara in Sanhedrin 65b explains that this refers to someone who starves himself and then sleeps overnight in a cemetery so that the impure spirit should rest upon him.

In contrast, merely speaking to the dead does not seem to be prohibited. We find in Berachos 18b that Shmuel went to the cemetery to ask his father, after he had died, where the money that had been deposited with him was located. We also learn in Taanis 16a that on fast days they used to go to the cemetery and one of the reasons given for this is so that the deceased buried there should request mercy on us. This is the source of the custom, mentioned in Shulchan Arukh Orach Chaim 559:10 in Rema, that on Tisha b'Av afternoon one goes to visit the graves. However, see Mishneh Berurah 581:27 that the idea of praying in a cemetery is because this is the place where the righteous rest and therefore our prayers are accepted better there. One should not pray to the deceased but should ask Hash-m that he should give us mercy through the merit of those who rest in the dust.

I think it should also be pointed out that most people are not on a sufficiently high spiritual level to have much meaningful contact with the deceased and that to dabble in this too much can be unwise for the average person. We see in our account concerning Rav Achai bar Yoshiyah that the workers who encountered his remains were not able to talk to him and it was only the great Rav Nachman who was able to communicate.

Kol Tuv and Shavua Tov

Dovid Bloom