-48V is the standard power supply
voltage for telco equipment, it has (almost) nothing to do with the transmitted signals. AFAIK the only time high(ish) voltages are used over the lines
is to send the ring signal on analog voice lines (for compatibility with legacy telephones having electromechanical ringers).Edit
: 48V signaling at the sort of frequencies used for T1 transmission would create an enormous EMI problem as well.Edit 2
: Speaking of legacy equipment, I remember reading somewhere that the telcos have maintained backward compatibility with analog phones going back more than 100 years. You can theoretically take an antique crank-style phone, splice a modular plug onto it, and plug it into your present-day POTS
phone line. Turning the crank will get you an operator, because it generates an outgoing signal on the line that is close enough to dialing '0' on a rotary dial phone for the equipment at the switching center to recognize it!
The years just pass like trains. I wave, but they don't slow down.
-- Steven Wilson