There are only correct and incorrect. The job of a pair of modems then is to receive digital data at one end, convert the data to sound, transmit an approximation of the sound, and recover an exact copy of the original digital data from the approximation of the original sound. The modems must transmit the data perfectly in a form that is trasmitted only approximately. Changes for the better The role of a telephone isn't just sending sound, it's sending the series of changes in sound that make up speech. The changes in speech are, for the most part, rather large changes, so even though the telephone does not reproduce them exactly, we can understand what is said.
Modems also use changes in sound, to transmit digital data.
Very old modems used large changes to transmit data. A 28.8Kbps modem runs 12 times faster by using subtle changes to represent 12 bits at a time. These changes are far smaller than the large changes making up most of speech. They are nuances of change at the very limit of what telephones are capable of. To use such nuances requires a telephone connection that can reproduce these subtle changes at the remote end and one that does not introduce any extraneous changes of its own.