Old phone signals (Plain old telephone service or POTS) cut off frequencies above 4 khz. You cannot hear the difference between f and s, as these are higher frequency sounds at around 8 to 12 khz. You'd have to say f like in Fred or s like in Steve? Cause you literally could not hear the difference.

This is the reason broadband internet (ADSL) is called broadband. Because dial up internet used the POTS frequency band below 4 khz, and broadband used the (broad) frequencies above.

It's also the reason you'd have to get a technician out to your house in order to get ADSL. They would install a frequency cutoff filter on the phone line into your house, and hookup the landline to the below 4 khz side and the ADSL modem to the above 4 khz side, so the POTS signal would not interfere with the ADSL signal.