[asterisk-users] screeching noise on zap channels

[Col Ferguson]

I have been working on a system where sometimes the users get a horrible
screeching noise that gets louder, typically as a call is hung up.

The system has 2 analog astribank channels with 8 FXO and 56 FXS ports, and
2 SIP phones, Asterisk 1.4.18, Zaptel 1.4.9.2. It is using the oslec echo
canceller.

In my testing, I have only experienced it on hang up of a call, but some of
the users say they have heard it at other times. It sounds like feedback
building up. The Receptionist said she had taken about 40 calls this
morning, and had only heard it once.

Because of the nature of the noise I have heard, I suspect something with
the echo cancelling, but I have no idea where to start looking. Maybe it
would just be easier/quicker to try a different echo canceller.

There is no noticable echo during conversations.

No one has heard the noise between the 2 sip phones, but as it seems
intermittent, this may be irrelevant.

Any hints as to how to track this noise down ?

Thanks,
Col