Method and apparatus for transmitting messages between communication devices via a communication channel allowing at least voice messages to be transmitted, the messages including data subdivided in a sequence of basic data units, the method including: a) encoding the basic data units as unique signals (us.sub.j) including a predetermined number of basic signals (b.sub.i), each basic signal having a unique fixed frequency (f.sub.i); b) transmitting a sequence of unique signals (us.sub.j) during a sequence of fixed time periods (t1, t2, t3, . . . ); c) receiving and decoding the sequence of unique signals (us.sub.j) into the sequence of basic data units; the data being transmitted in superposition over voice.