If the loop connected from a line card to a telephone were extremely long, then reflections from the far end of the loop could be thought of as echoes. Generally, two wire loops are short enough so that the round-trip delay through a loop is negligible. Generally, the impedance of the two wire loops is lumped with the telephone and the effects of far-end reflections are included as part of the hybrid balance. Another source of echo is acoustic echo which is echo due to coupling from the speaker to the microphone of a telephone set. This path can once again be lumped with hybrid balance.
Speakerphones have the potential of much longer delay paths and low loss, which can create echo.
Due to the potential of highly time-variant echo paths, long delay, and multiple echo paths, echo control for speakerphones is extremely difficult. Echo control is most commonly achieved by turning off the TX path when RX is active or vice versa. Echo control for these types of devices is normally done at the device itself. It's possible to correlate the signal sent in one direction to the signal being received from the other direction, and determine the amount of echo. It is also possible to dynamically adjust the tap weights in a digital filter to subtract a delayed portion of the signal being sent from the signal being received, so as to minimize the echo signal.