[cisco-voip] FXO/Analog line hum - 60Hz - Noise

[Carter, Bill]

In a post on "Echo Cancellation and 7911 Phone Loads", Wes's answer
included: "Examples of noise include any slips or errors on T1's and
electrical cables (60Hz, fluorescent lights especially) located too
close to analog (FXS/FXO) connections."

I have a customer with Analog lines connected to FXO ports with constant
hum. If I have a call from an IP Phone out the FXO ports, put both
phones on mute, both phones still hear hum.

The analog lines run from outside the building, through a dropped
ceiling, into the wiring closet. The wiring closet is actually a room
with the AC blower/furnace and hot water heater.

I have suspected electrical interference for awhile. They sometimes have
a weird event where users will suddenly hear a screeching sound on the
phone. It will last for 1-2 minutes. My theory is this happens when the
blower on the AC/furnace kicks on. I have been able to have a user tell
me if the screech happens at the same time the blower kicks on.

How can I check for 60Hz interference? Is there an electrician tool I
can connect to the analog line to see this? Someone suggested adding an
analog line DSL filter to the lines.