Question : Humming sound when making a conference call?

We have an Avaya IP office system, with a PBX. My colleagues are currently on a conference call and there's a humming sound coming from the phone. It sounds like some interference, but we've not been able to figure out what's causing the sound. Any ideas will help.

Answer : Humming sound when making a conference call?

Is someone's cell phone too close to the phone unit?