17. A data processing apparatus comprising: inputting means for inputting alphanumeric character data representing a message to be transmitted; storing means for storing the alphanumeric character data input by said inputting means; displaying means for displaying said alphanumeric character data stored in said storing means; and means for causing said displaying means to display first information indicating to a user, key operations of a push-button telephone corresponding to said alphanumeric character data stored in said storing means so as to input a message corresponding to said alphanumeric character data by the user performing said indicated key operations of said push-button telephone connected to a public telephone network means; and wherein said causing means includes converting means for converting said alphanumeric character data stored in said storing means into said first information indicating a series of key operations of the push-button telephone, based on a preset data conversion format.
22. A data processing apparatus according to claim 17, wherein said converting means comprises means for converting each alphanumeric character data stored in said storing means into two-digit numeric data.