[asterisk-users] Static noise on bridged calls to PSTN, although the trunk line is clean on its own

[Sebastian Arcus]

I have two setups with SIP hardware phones as extensions and POTS lines 
as trunks. Internal SIP to SIP calls are crystal clear, but all calls 
bridged to POTS have a significant amount of static noise. The problem 
is that if I plug a POTS phone directly into the line, there is almost 
no static noise - the line is clean. It's like Asterisk (or the 
hardware) amplifies the static noise. What I've tried so far:

1. Connect Asterisk with a short cable directly into the master phone 
socket, where it enters the building.
2. One of the lines carries ADSL - so I double filtered it.
3. Tried three different phone sets (one Grandstream, two Cisco models).
4. Tried an OpenVox A400P PCI card and a Sangoma U100 USB adapter as 
analogue-to-digital interfaces.
5. Reduced the software echo canceller in chan_dahdi.conf to 32 and even 
16 - until I could actually start to hear echo. Still no difference.
6. Reduced the rxgain and txgain in chan_dahdi.conf to 0 - but the 
static noise is still there.
7. Tried different phone cables for the pots line.
8. Tried a different motherboard on the computer with Asterisk and 
checked there is no IRQ sharing. Tried when there was no other load on 
the Asterisk computer.
9. Tried Asterisk 1.6, 1.8 and 10

Is there anything else I can do - or should I just give in to the static 
noise? Is that how other hybrid setups work - do you get static noise on 
the line - more than if plugged directly? The client is adamant that the 
noise on the line is too high - by comparison with the quality on mobile 
phone calls (which are digital, incidentally) - so if I don't find a 
solution, I suppose I will just have to rip it all out and let one of 
the companies with proprietary phone systems install one.

Any hints appreciated.

Sebastian