Hi,
what you are looking for is a voice band modem (or voiceband modem).
There are many modulation systems and techniques for this purpose. Standards used universally over telephone lines are ITU-T reccomendations (V.21, V.22, V.26, etc.). Using sophisticated techniques, modems acording to V.34 standard allow 33600 bps over analog (less than 4 kHz bandwidth) channels.
Binary FSK is the simplest modulation/demodulation system for this purpose. Speed limitation is bandwidth. Standard modems for nominal telephone lines bandwidth give 1200 bps with FSK. For more speed, other modulation methods are used.
If you have an audio channel of enough bandwidth you can increase speed over the 1200 bps using Audio FSK. For example, if you want 4800 bps you would need on the order of 10 kHz of bandwidth.
Regards
Z