IP Office 9.0(829) in Basic mode with DIGSTA8/ATM4 in slot 1, and COMBO6210/ATM4 V2 in slot 2
12x 1416 Digital sets
5x POTS lines provided by Frontier

The problem is that echo randomly occurs on all lines, inbound or outbound calls. Only the person on the external/POTS side hears their voice echoed back. The local user on the IP Office side NEVER hears any echo at all and there is zero echo on internal calls.

I have tried doing auto impedance on all lines, they all come back with the same value (900+2150nF), which did not solve the problem.

The echo does not occur using a analog phone on the same lines.

Any tips?