I have a customer that has a 550.4400 and they intermittently have an electrical buzzing sound on the phone. I have swapped phones, replaced station card, used line filters and this noise still returns. The station cable is STP because of RF issues in the area and have moved the port to different cable pairs with no change. There is another extension that is on this cable and it works great....go figure, any ideas.

If it's sitting next to a computer monitor, move it!

Had this happen to me, customer had phone under a small floursecent desk lamp. Only buzzed when lamp turned on, drove me nuts. Check for the new green lamps also.

Does this happen on outside calls, intercom calls or both? Does this happen on speaker phone, handset call or both? The reason I ask is, some of the older model phones had bad microphones and any call to that phone while on speaker would give the other party a buzzing noise.