One customer complained that there always had echo and noise on the phone, loud squeaking and squealing, and heard faintly sometimes.

They said it happened on all extensions, both inbound and outbound.

In the Analogue Options,
Impedance is set to Automatic,
Echo Cancellation is set to ON,
Echo Reduction is set to ON.

I am going to put line keys on the phones to find out which line/lines have the issue, then to test all PSTN lines to find out whether it's line quality issue.

Any other suggestion?

What kind of phones, digital or IP?
What kind of trunks?

Sorry, forget some details:
System is IPO 500 v2 with 2 combiantion cards, one is ATM4, the other one is BRI.

Extensions are digital 1416, only PSTN Line are in use.

It's not likely happening on the BRI's, maybe the analog lines from the Telco are out of whack...

BRI is not in use, so I cannot say whether they are OK or not.

It happened sometimes, not always, When I made some calls to them. In my opinion, maybe some of the PSTN lines have some issues.

But I need to prove that nothing wrong with the phone system!

Can you loan the customer a PBX or another IP Office to prove the POTS lines

Have you run the Auto impedance test?
I would start there.

I would check the POTS lines at the provider Demark with everything inside disconnected.
Also I assume you have the IPO properly grounded?

Auto impedance test? Is that Telstra's job?

the system is not provided by us, we just programmed and installed it.

I am sure the system is not grounded, so I will do it next.

And I turned "Echo Reduction" off on all lines, not heard any feedback yet.

Thank you for your advices!