Which of the following problems is most likely caused by acoustic coupling between the earpiece and
microphone in an IP phone handset when an IP phone user places a call to a PSTN user? (Select the best
answer.)

A.
The PSTN phone user hears echo.

B.
The IP phone user hears echo.

C.
The PSTN phone user experiences delay.

D.
The IP phone user experiences delay.

Explanation:
The public switch telephone network (PSTN) user hears an echo when there is acoustic coupling between the
earpiece and the microphone in the IP phone user’s handset. Acoustic coupling occurs when sound from a
local user’s handset or headset speaker is picked up by the same device’s microphone, which then transmits
that sound to the remote user. This causes the remote user to experience an echo.
An IP phone user will hear an echo from telephone hybrids on the PSTN. The PSTN typically uses a four-wire
method of transmitting bidirectional audio. However, homes and small businesses with analog devices often
combine audio signals on two wires. A telephone hybrid converts between the two systems.
Delay, or latency, has a number of causes on both the PSTN and IP networks. Contributors to delay are
conversion from analog to digital or viceversa, buffering, codec conversion, and more. However, acoustic
coupling is more likely to cause echo for the remote user than delay.

https://www.cisco.com/c/en/us/support/docs/voice/voice-quality/19640-echo-ipphone-gateway.html#pstnecho