No modulation method in use today actually does this. It's not practical due to the relatively long time it would take to detect all 3 types of changes. The main problem is that frequent shifts in phase can make it appear that a shift in frequency has happened when it actually didn't.
To avoid this difficulty one may simultaneous change only the phase and amplitude (with no change in frequency). This is called phase-amplitude modulation. It is also called quadrature amplitude modulation (= QAM) since there were only 4 possible phases (quadrature) in early versions of it. This method is used today for the common modem speeds of 14.4k, 28.8k, and 33.6k. The only significant case where this modulation method is not used today is for 56k modems. But even 56k modems exclusively use QAM (phase-amplitude modulation) in the direction from your PC out the telephone line. Sometimes even the other direction will also fall back to QAM when line conditions are not good enough. Thus QAM (phase-amplitude modulation) still remains the most widely used method on ordinary telephone lines.
