generaly it is a nuce feature to have as echo is realy anoing.
you need echo cancellation on the card to remove echo comming form the PSTN as analog phone lines can in some cases generate a lot of echo
especialy if the calibration is bad.
there is a number of ways that echo can acurre.
1 long analog PSTN lines (electric signal bouncing inside the cabel)
the longer the cabel the worse the problem
2 badly designed phones (most phons always play a littel of what you say in your headset so you can heare that you are talking, but some will also work the other way as what your heare is also transfreed to your mic, the speaker phone afect)
3 software bugs, (huu yes I know it but I have it often with my skype2skype that there is a 1-3 sec echo whre I’m talking with my self???)
Debian / Ubuntu fann