Is it just on one make/model of set, just one extension? Are they all muting using the same method...a mute button on the headset? Are all headsets the same make?What make? If they were using a handset, is there a mute button on the set, or is this only a function on the headset?
Trying to determine if the set is assuming the call has ended because of a condition the headset is presenting when muted. Perhaps a headset that is not fully compatible with that phone.
Can you duplicate this when calling another extension?