When receiving a conversion instruction from a key input section, a CPU reads character data stored in a message memory to a conversion buffer, converts the same into numeric data base on a conversion table, causes a tone signal generator to convert the same into a DTMF signal and outputs the signal...http://www.google.com.au/patents/US5583921?utm_source=gb-gplus-sharePatent US5583921 - Data transmitting/receiving apparatus and data communication system
