Copper wire has a lot of bandwidth, but traditional phone lines had low-pass filter at 4KHz, so anything above is cut off. Between switching stations signal is modulated and multiplexed, but on the receiving end it is again restored to the original analog signal. Therefore fax-machines couldn't possibly expect anything on 56KHz for sure, it would never get to them. Also, faxes make that well-known noise when establishing connection, and if it were around 56KHz, we wouldn't be able to hear it - anything above 15KHz is very hard to hear for adults. So the OP probably meant 56kbps as a transmission rate, which again is not true because faxes existed much before 56K was possible and many worked on as low rates as 2400bps.