I believe the feature you are trying to disable is MOH (Music/Messages on hold). The BGM (background music) is a personal user option which controls playback of an audio source on the individuals telephone speaker. To turn off or disable MOH usually requires system administration access via a programming interface (admin phone or computer software). I am not too familiar with the Inter-Tel systems and refer all my local work to John.

On thing you might try if you really need the MOH removed is to locate the phone system (probably mounted on a wall in a service closet or data room) and then identify the MOH source which is usually an external device plugged in to the phone system via an audio cable similar to that of a headset jack or RCA cables. Keep in mind, with the source removed and not replaced, outside callers will only receive silence when placed on hold and could come to the conclusion that they have been hung up on rather than placed on hold. Good luck!