Callers hearing and Echo

PBX Firmware: 4.211.64-5
PBX Service Pack:
SNOM phones
Incoming lines from Vitelity

Lately, callers have been complaining of an echo on the phone.
What strategies exist for dealing with this?

I’ve looked at several long involved websites that discuss highly technical aspects of echo – I’m not that deep.

Can you refer me to some places where I might find a solution?

Thank you!