|Speed has nothing to do with it. It is the communications method used.|
A 56k Analogue modem uses frequencies of up to 3.5khz, right in the middle of tha audio range which the telephone system was designed for. The fax protocol converts its information into tones within that frequency range. Any Fax Modem must be capable of at least 9 kbs. Some fax protocols use 14 kbs. A lot slower than the 10 Mbs that a DSL Modem is capable of.
An DSL modem uses frequencies up to 100 Khz and not less than 25kz. 25kx it outside of the audio range and way outside anything a 56k Modem could produce. Therefore a DSL Modem cannot support the fax protocols which as stated above, uses frequencies within the audio range. These high frequencies are only possible because the special equipment installed at the telephone exchange where the DSL frequencies and audio frequencies are separated and sent their separates ways.
It is these higher frequencies that make it possible to use Broadband and consequently much higher data rates, and carry out a telephone conversation simultaneously over the same pair of copper wires. You cannot do that with a 56k dial up Modem.