Harry Dresden uses a speaking stone to communicate with Ebenezar McCoy. The spell to activate it is fairly delicate magic. Ebenezar McCoy crafted the speaking stones and has one identical to Dresden's.
Dresden's stone is a smooth piece of fire-rounded obsidian. He keeps it in a cigar box amid some other miscellaneous stones as camouflage.
The magic used with the speaking stone is very much like the magic that Dresden and Elaine Mallory used to talk to each other mind-to-mind. Dresden uses that same magic again to talk to Susan in the Temple on the pyramid at Chichén Itzá.
Dresden uses it with delicate spell-work inside his copper circle in his lab. When the spell activates, the lab vanished into inky darkness while the energy field of the circle he sat in became a cylinder of pale blue light from the floor into infinite space. While talking, the voice echoes oddly. It's the same effect for the person on the other end. The two parties can see each other plainly.
In the series
In Changes, Harry Dresden uses it to communicate with Ebenezar McCoy. Dresden asks him if the Grey Council can help get information for him on where the Reds may have taken the girl. Before Dresden can tell him the truth that Maggie is his daughter, Ebenezar cuts him off by muting his voice and scolds him for only thinking of only one child, and tells him to meet at Toronto safe house in twelve hours, then hangs up.
Later, McCoy calls him on the Stone while Harry Dresden is in the Leanansidhe's limo with the rest of his team on their way to the Ways to go rescue Maggie from Arianna Ortega and the Reds at Chichén Itzá. The team thinks Dresden's acting odd when the call comes in. Ebenezar starts scolding Dresden for missing the meeting and for what he perceives as foolishness. Dresden finally manages to tell him that Maggie is his daughter and that Arianna Ortega plans a Bloodline curse to kill her, Susan and himself. Ebenezar softens to Dresden's pain and sees the gravity of the situation. He tells Dresden to go get his girl.
Though it's never stated, it can be presumed that it is at least possible that other unknown members of the Grey Council have their own speaking stones.