Vox: A Novel (1992) by Nicholson Baker, is written almost entirely as a verbatim telephone conversation between a man and a woman, separated by both geographical distance and time-zone. Their conversation ostensibly begins in a telephone chat-room, and after spending some time in a semi-public forum, they switch to a private line to continue their discussion.
I found this novel incredibly erotic, as this couple of strangers begin by describing themselves and their attire, before moving on (cautiously) towards a frank discussion of sexuality, eroticism and mail-order underwear.
The ending? Without meaning to ruin this terrific read (best done in one session) for anyone else, the ending is... well, happy.