Polycom VVX 410s and Snom 710s
FreePBX 13.0.192.8
Asterisk 13.15.0
PBX Firmware:10.13.66-20
DAHDI Version: 2.11.1 Echo Canceller: HWEC
Added an A200, and routed calls over that, and I’m getting a little whine. I don’t hear it when I’m on a regular analog handset. Is this something that might be setting related? No DSL or anything on this line.
I’m getting the same thing on some of my Analog connections (using different hardware, but HWEC). I’ve tried tuning the EC and changing the gain, both of which helped a little bit, but I can still hear it if I’m looking for it.
How did you tune the EC and gain in FreePBX? Or did you do something card specific?
I’ve tried lots of different settings. Turning the gain down helped the most, but you have to be careful not to go too low.
To me, that sounds like it could be a 60/50 Hz hum, typically experienced when unshielded cable comes into close proximity to power sources that are radio-logically noisy … Do you have any copper connections that could possibly be subject to interference? Maybe your PSTN connections are laying too close to some power supplies that lay between you and the phone company interface. I would also recommend checking the telephone cables themselves. CCA (Copper Clad Aluminum) cable is probably the absolute worst thing to use for critical applications and is commonly sold in discount stores as a cheap patch cable, sometimes as a ‘flat’ cable or has clear jacketing. I would recommend using a set of STP (Shielded Twisted Pair) cables if the pathway crosses interference sources that absolutely cannot be avoided.
If you replace the cables and the hum remains, I’d recommend checking your connections inside the NetBorder to verify no power connectors lay on or against said A200 card. They are sensitive snowflakes 
Always look at running fxotune before you mess with gains or echo correction, probably the biggest cause of echo ona POTS line is a badly balanced 2-4 wire hybrid, also 50/60Hz buzz is likely a grounding problem, only ground one end of your tranmissin line,
ref:
man fxotune
then google fxotune, 2-4 wire hybrids, compex transmission lines, the work that Bell labs (AT&T) have done over the last 150 years 
(sorry, they made us all learn applied mathematics in my UK Grammar school, it was eventiually useful) )i
In my example, the whine is around 8000 Hz, so power line noise was ‘pretty much’ ruled out.
Interestingly
is a cross product of bad g711 sampling and bad hardware filtering , Speech filters for G711 should be quite sharp and only band-pass 350 to 3500 Hz with a very sharp attenuation outside , nyquist says that 8000Hz sampling plus the normal FFT hardware filter suggests that maybe you need to get the kettle off the stove and make your tea or get rid of your ground loop, both should be at least 18db down
Can it be heard permanently? From the very beginning of the call? You might have some DC offset on the line.
2/4 wire hybrids are intrinsically immune from “DC offfset”, both the L and C of the the “bridge” prevent that, ( ref:- Bell Labs, circa 1920), don’t waste your time there 