Have you ever been on a call and heard your words echoed back to you? Annoying and as frustrating as it is, it is actually present in all calls. In order for echo to be noticed it needs to be loud and delayed. In most PSTN lines the echo is loud however not delayed. On cell phones it is loud and delayed and the same can be true with VoIP calls.

Another reason for echo is a signal mismatch from a 2 wire connection to a 4 wire connection (Hybrid). That causes a impedance mismatch (See below).

VOIP and Echo – Voice over IP calls don’t actually create additional echo, but they also have a delay budget in the range of 150 ms to preserve audio quality, so VoIP systems commonly employ echo cancellation as well. In order to understand echo cancellation and the metrics that go along with it, we need to look at some other aspects of echo that affect the implementation of echo cancellation on voice gateways.

Strictly speaking, echo is never caused by VoIP. In fact, what happens is that the longer delays introduced by all VoIP systems reveal echo that was imperceptible with the shorter delays of the PSTN. By delaying existing echo signals longer, the VoIP network causes them to fall outside that 25 ms window and so become perceptible to callers.

One thing to always remember is Echo is Never Digital – It is always caused by the analog component of the Telephone System. The digital stream of packets traveling in one direction of a VoIP call cannot “bleed into” the digital stream of packets in the other direction, nor are the packets played back at the receiving end of the call. The same is true for the digital parts of the Public Switched Telephone Network (PSTN): while the underlying electrical signals carrying the bits over the traditional switched telephone network are, indeed, analog, the corruption of those signals results in digital noise or other problems, but not in echo.

Types of Echo

• Hybrid Echo – Hybrid echo is caused by an impedance mismatch in a hybrid circuit, such as a two-wire to four-wire interface, which allows the Tx signal to appear on the Rx path. At several places along a phone circuit, your voice can get into the return channel and come back to you. The first interface where echo may occur is at the transition between a 4-wire and a 2-wire interface. Analog telephone handsets are 2-wire devices. At some point in the path, perhaps in a local PBX, there is a hybrid interface that converts the network 4-wire interface to the 2-wire interface. Impedance mismatches here will reflect some of the energy back into the network, creating a potential source of hybrid echo.

• Acoustic Echo – Another common source of echo is the basic hardware: the mouthpiece of the phone at the far end may be too close to the earpiece, or it may be poorly insulated, so that your voice is heard and forwarded on the same return channel as the one on which the person at the far end is speaking. Therefore, the analog phone itself is a possible source of acoustic echo. But even more suspect these days is the speaker phone function of the phone at the far end of the call. Speaker phones broadcast the voice and simultaneously listen to the voices of the speakers in the room. Despite advanced DSP functions, it is very easy for speaker phones (especially cheap ones) to send back some of the far end voice as part of what they are “hearing.”

If you ever find yourself troubleshooting echo issues in a VoIP environment save yourself some time and follow this steps:

1. If all phone are experiencing the issue, be sure that the Analog Gateway has Echo Cancellation enabled. 
2. Check with the PSTN Provider and have them check the circuit signal levels. (Very Common)
3. If one phone – Check handset cord (2 wire to 4 wire) and
4. If headset – Check base settings and cord.

More can be found at these links

http://www.voip-info.org/wiki/view/Causes+of+Echo

http://www.ca.com/us/~/media/Files/whitepapers/keys-to-minimizing-echo-in-voip-networks.pdf