I did a search on Google, and "telephonic conversation"
got 200,000 odd hits, and "telephone conversation"
got about three million hits.
It looks like both expressions are in use. In point of fact, Mark Twain has written a book entitled "Telephonic Conversation"
(http://www.goodreads.com/book/show/6601 ... nversation
May I know whether "telephonic conversation"
is acceptable in modern English?