Now look closely at Mark 12:26-27:
“But concerning the dead, that they rise, have you not read in the book of Moses, in the burning bush passage, how God spoke to him, saying, “I am the God of Abraham, the God of Isaac, and the God of Jacob’? He is not the God of the dead, but the God of the living. You are therefore greatly mistaken.”
Look at the context. God is the God of the living, the spiritually living. Those who denied the resurrection were challenging Jesus about the absurdidty of marriage as it pertains to the Levitical law of brothers who died without issue. Marriage belongs to those alive on Earth, not in Heaven. The context has NOTHING to do with communicating across the life/death boundary.