In the past I used to look at Echo Suppressors and Echo Cancellers (EC).
Essentially, for the PSTN you have points where the Two Wire Circuit (Local-Loop)
is connected to a Linecard (SLIC/SLAC) so that a Four wire circuit is created for
RX / TX connectivity. As a coil is used there is a chance for echoes to be generated as
the Line Circuit is “not balanced”.
Using an Echo Canceller with your circuit you can inject
a signal to “dampen” the Echo. Good step is to look at your signalling routes and to configure via signalling the use of an EC.
VBR/ Wallis Dudhnath