Core Issue
The echo is caused by an impedance mismatch with the local Telco.
Resolution
An impedance of 600 ohms real is standard for most Telcos and PBXs. Therefore, a problem with impedance mismatches is rare.
However, there are differences. For example, some PBXs use 900c. If the impedance is set incorrectly (if there is an impedance mismatch), a significant amount of echo is generated. This echo could be masked if the echo cancel command is issued.
In addition, gains might not work correctly if there is an impedance mismatch. You can configure impedance in Cisco IOS Software by issuing the impedance voice port command.
Problem Type
Voice quality problems (Audio quality: Choppy voice, hissing, echo, etc.)
Voice Quality
Echo (Listen to short or long echo samples.)
Frequency and Scope of Symptom
Occurs intermittently
Occurs only on some calls
End Where Problem is Heard or Device Type Involved
Both ends
Call Topology and Direction
IP network > PSTN/PBX/gateway > IP network
PSTN/PBX/gateway > IP network > PSTN/PBX/gateway
TDM Signaling Type
Analog
