[Asterisk-Users] Echo cancellation

[Kristian Larsson]

I am having problems with echo, first let me
explain my setup:

I have a Gateway box, which basically is an
Asterisk with a PRI card. It's only job is to
interface with 2 incoming ISDN PRI connections.
Then there are two other asterisk boxes to which
my users are registered.
Dialing from a phone it hits the first asterisk
which forwards it to the gateway box and then on
to the PSTN.

What are the general causes of echo?
When calling from my SIP phone I hear no echo but
the other end, the PSTN end, hears a lot of echo.
What could cause this?

  Kristian.