Telepaths can Transmit messages directly into other elves' minds, but only other Telepaths can respond in kind. Often, when one is a Telepath and the other is not, the Telepath will simply read the other's thoughts after the non-Telepath consents. Sophie Foster is the only known Telepath to be able to transmit into animals' minds.
Sophie transmitted to Fitz from the Four Seasons Tree in Keeper of the Lost Cities. In Exile, Fitz was able to breach Sophie's mental blocking and transmit to her due to Sophie's abilities being "broken". At the end of the book, Mr. Forkle healed her mind, and Fitz lost the ability to transmit to her. At the end of Everblaze, Mr. Forkle showed Fitz how to enter Sophie's mind, and he regained the ability to transmit to her. The special edition of Flashback revealed that Fitz transmits "it's me" so that Sophie's subconscious will let him into her mind.